Pythagoras and Trigonometry: Edexcel 4MA1 Past Paper Questions and Worked Solutions
This page collects every pythagoras and trigonometry question from the Pearson Edexcel International GCSE Mathematics A (4MA1) Higher tier Paper 1H and Paper 1HR, January 2020 to June 2025: 34 questions worth 153 marks. Questions run from easier to harder. Try each one first, then click “Show solution” for a step-by-step method with the reason for every step. All answers are checked against the official Pearson mark schemes.
Here is a right-angled triangle with hypotenuse 52 cm, one shorter side 30 cm and the other shorter side h cm. Calculate the value of h. Give your answer correct to 3 significant figures.
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- The 52 cm side is opposite the right angle, so it is the hypotenuse. To find a shorter side, subtract the squares.
- h2 = 522 − 302 = 2704 − 900 = 1804
- h = √1804 = 42.47…
Answer: h = 42.5
The diagram shows triangle ABC with a right angle at B. AC = 8.4 cm and angle ACB = 65°. Work out the length of the side AB. Give your answer correct to 3 significant figures.
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- AC is the hypotenuse (opposite the right angle). From the 65° angle at C, AB is the opposite side, so use sin.
- sin 65° = AB8.4, so AB = 8.4 × sin 65° = 7.612…
Answer: 7.61 cm
The shaded shape is made using three identical right-angled triangles and a square. Each triangle has hypotenuse 16 cm. The triangle on top of the square has a vertical side of 12.8 cm and its base is the top of the square. The other two triangles share a horizontal side and fit against the right-hand side of the square. Work out the perimeter of the shaded shape.
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- The top triangle has hypotenuse 16 cm and one side 12.8 cm. Use Pythagoras for its base: √162 − 12.82 = √256 − 163.84 = √92.16 = 9.6 cm
- So every triangle has sides 9.6, 12.8 and 16, and the square has side 9.6 cm.
- The bottom-right triangle’s vertical side is the right side of the square (9.6), so its horizontal side is 12.8. The top-right triangle shares that 12.8 side, so its vertical side is 9.6.
- The top triangle’s vertical side is 12.8, but 9.6 of it is covered by the top-right triangle. The exposed part is 12.8 − 9.6 = 3.2 cm.
- Go round the outside: left of square 9.6 + bottom of square 9.6 + three hypotenuses 16 × 3 + exposed edge 3.2
- 9.6 + 9.6 + 48 + 3.2
Answer: 70.4 cm
The diagram shows a shaded shape AEBCD made by removing triangle AEB from rectangle ABCD. AE = 7.2 cm, BE = 5.4 cm, BC = 6 cm and angle AEB = 90°. Work out the perimeter of the shaded shape.
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- The perimeter of the shaded shape is AE + EB + BC + CD + DA. We know all of these except CD.
- CD = AB (opposite sides of a rectangle). Find AB with Pythagoras in the right-angled triangle AEB: AB2 = 7.22 + 5.42 = 51.84 + 29.16 = 81, so AB = 9 cm
- DA = BC = 6 cm
- Perimeter = 7.2 + 5.4 + 6 + 9 + 6
Answer: 33.6 cm
Here is a right-angled triangle PQR with the right angle at R. PR = 24.3 cm and angle RPQ = 63°. Calculate the length of PQ. Give your answer correct to 3 significant figures.
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- From the 63° angle at P: PR is the adjacent side and PQ is the hypotenuse, so use cos.
- cos 63° = 24.3PQ, so PQ = 24.3cos 63° = 53.52…
Answer: 53.5 cm
The diagram shows isosceles triangle ABC with AB = AC = 17.5 cm and BC = 28 cm. Calculate the area of triangle ABC.
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- The line from A down to the middle of BC is the height, and it splits BC into 14 cm and 14 cm.
- Pythagoras: height2 = 17.52 − 142 = 306.25 − 196 = 110.25, so height = 10.5 cm
- Area = ½ × base × height = ½ × 28 × 10.5
Answer: 147 cm2
The diagram shows an isosceles triangle ABC. AB = 7 cm and AC = BC = y cm. The area of the triangle is 42 cm2. Work out the value of y.
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- Area = ½ × base × height: ½ × 7 × h = 42, so h = 12 cm
- The height from C cuts the base in half (the triangle is isosceles), making a right-angled triangle with sides 3.5 cm and 12 cm.
- Pythagoras: y2 = 122 + 3.52 = 144 + 12.25 = 156.25
Answer: y = 12.5
ABCD is a trapezium with one line of symmetry. Angle ADC = 60°, AD = 12 cm and DC = 47 cm. Work out the area of the trapezium. Give your answer correct to 3 significant figures. Show your working clearly.
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- Draw a perpendicular from A down to DC. This makes a right-angled triangle with hypotenuse AD = 12 cm and angle 60° at D.
- Height of the trapezium = 12 × sin 60° = 10.392… cm
- Horizontal distance from D to the foot of the perpendicular = 12 × cos 60° = 6 cm
- The trapezium is symmetrical, so the same 6 cm is cut off at the other end: AB = 47 − 6 − 6 = 35 cm
- Area = ½(AB + DC) × height = ½ × (35 + 47) × 10.392… = 426.08…
Answer: 426 cm2
The diagram shows triangle PQR with a right angle at Q. PQ = 6.5 cm and angle PRQ = 24°. Work out the length of QR. Give your answer correct to 3 significant figures.
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- From the 24° angle, PQ is the opposite side and QR is the adjacent side, so use tan.
- tan 24° = oppositeadjacent = 6.5QR
- QR = 6.5tan 24° = 14.599…
Answer: 14.6 cm
Here is isosceles triangle ABC with AB = CB. D is the midpoint of AC and DB = 16 cm, with DB perpendicular to AC. Angle DAB = 65°. Work out the perimeter of triangle ABC. Give your answer correct to one decimal place.
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- In right-angled triangle ADB: DB = 16 is opposite the 65° angle.
- AB (hypotenuse): sin 65° = 16AB, so AB = 16sin 65° = 17.653… cm
- AD (adjacent): tan 65° = 16AD, so AD = 16tan 65° = 7.460… cm, and AC = 2 × 7.460… = 14.921… cm
- Perimeter = AB + BC + AC = 2 × 17.653… + 14.921… = 50.22…
Answer: 50.2 cm
R and T are points on a circle, centre O. RT = 12 cm. M is the midpoint of RT. Angle ROM = 52°. Work out the area of the circle. Give your answer correct to 3 significant figures.
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- OM joins the centre to the midpoint of a chord, so OM is perpendicular to RT (the perpendicular from the centre to a chord bisects the chord). Triangle ROM has a right angle at M.
- RM = 12 ÷ 2 = 6 cm, and RM is opposite the 52° angle. OR is the hypotenuse (the radius).
- sin 52° = 6r, so r = 6sin 52° = 7.614… cm
- Area = π × 7.614…2 = 182.13…
Answer: 182 cm2
The diagram shows trapezium ABCD in which BC and AD are parallel. The trapezium has exactly one line of symmetry. BC = 8.4 cm, AD = 17.6 cm and the area is 179.4 cm2. Work out the size of angle ABC. Give your answer correct to 1 decimal place.
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- Area = ½(BC + AD) × height: ½(8.4 + 17.6) × h = 179.4, so 13h = 179.4 and h = 13.8 cm
- The trapezium is symmetrical, so the bottom overhangs by the same amount on each side: (17.6 − 8.4) ÷ 2 = 4.6 cm
- Drop a perpendicular from B: a right-angled triangle with height 13.8 and base 4.6. tan A = 13.84.6 = 3, so angle A = 71.565…°
- BC is parallel to AD, so angles A and B are co-interior and add to 180°: angle ABC = 180° − 71.565…°
Answer: 108.4°
In the diagram, ABC and DAC are right-angled triangles (right angles at B and at A). BC = 6 cm and AC = 7.5 cm. The area of quadrilateral ABCD is 31.5 cm2. Work out the length of AD.
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- Find AB with Pythagoras in triangle ABC (AC is the hypotenuse): AB2 = 7.52 − 62 = 56.25 − 36 = 20.25, so AB = 4.5 cm
- Area of triangle ABC = ½ × base × height = ½ × 6 × 4.5 = 13.5 cm2
- Area of triangle DAC = 31.5 − 13.5 = 18 cm2
- Triangle DAC has its right angle at A, so AC and AD are the base and height: ½ × 7.5 × AD = 18
- AD = 36 ÷ 7.5
Answer: 4.8 cm
The diagram shows a roof support made from four lengths of wood, AB, AC, BC and MC. AC = BC = 9 m, AB = 12 m and angle AMC = 90°. Lewis is going to buy lengths of wood to make the roof support. The wood costs 21.50 euros per metre. Each length of wood he buys has to be a whole number of metres. Work out the total cost of the wood Lewis needs to buy. Show your working clearly.
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- Triangle ABC is isosceles (AC = BC), so the perpendicular from C meets AB at its midpoint. So AM = 12 ÷ 2 = 6 m.
- Use Pythagoras’ theorem in the right-angled triangle AMC, where AC = 9 m is the hypotenuse: MC2 = 92 − 62 = 81 − 36 = 45
- MC = √45 = 6.708… m
- Each length must be a whole number of metres, so round each length up (rounding down would make it too short): MC needs 7 m. AB = 12 m, AC = 9 m and BC = 9 m are already whole numbers.
- Total length to buy = 12 + 9 + 9 + 7 = 37 m
- Cost = 37 × 21.50
Answer: 795.50 euros
The diagram shows a vertical cliff of height 100 metres with a vertical radio mast on top of the cliff and a buoy in the sea at the point B, d metres from the base of the cliff. The angle of elevation from B to the top of the cliff is 20°.
(a) Calculate the value of d. Give your answer correct to 3 significant figures. (3)
The point A at the top of the radio mast is vertically above the top of the cliff. The angle of elevation from B to A is 25°.
(b) Calculate the height of the radio mast. Give your answer correct to 3 significant figures. (3)
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Part (a)
- In the right-angled triangle, 100 m is opposite the 20° angle and d is adjacent, so use tan.
- tan 20° = 100d, so d = 100tan 20° = 274.74…
Answer: d = 275
Part (b)
- Height from sea level to A: tan 25° = height274.74…, so height = 274.74… × tan 25° = 128.11… m
- Mast height = 128.11… − 100 = 28.11…
Answer: 28.1 m
The diagram shows parallelogram EFGH. EF = 9.3 cm, FG = 14.7 cm and angle EFG = 106°.
(a) Work out the area of the parallelogram. Give your answer correct to 3 significant figures. (2)
(b) Work out the length of the diagonal EG of the parallelogram. Give your answer correct to 3 significant figures. (3)
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Part (a)
- The parallelogram is two equal triangles, so area = ab sin C = 9.3 × 14.7 × sin 106° = 131.41…
Answer: 131 cm2
Part (b)
- EG is opposite the 106° angle in triangle EFG. Cosine rule: EG2 = 9.32 + 14.72 − 2 × 9.3 × 14.7 × cos 106°
- = 86.49 + 216.09 + 75.36… = 377.95…
- EG = 19.44…
Answer: 19.4 cm
The diagram shows two right-angled triangles, DEF and EFG. In triangle DEF, the right angle is at F, DE = 12 cm and angle DEF = 40°. In triangle EFG, the right angle is at E and angle EGF = 28°. Work out the length of EG. Give your answer correct to 3 significant figures.
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- In triangle DEF: DE is the hypotenuse and EF is adjacent to the 40° angle, so use cos: EF = 12 × cos 40° = 9.192… cm
- In triangle EFG: from the 28° angle at G, EF is opposite and EG is adjacent, so use tan: tan 28° = EFEG
- EG = 9.192…tan 28° = 17.28…
Answer: 17.3 cm
The diagram shows isosceles triangle EFG with EF = GF. Angle EFG = 130°. The area of triangle EFG is 74 cm2. Work out the length of EF. Give your answer correct to 3 significant figures.
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- We know two equal sides and the angle between them, so use area = ½ab sin C with a = b = EF.
- ½ × EF2 × sin 130° = 74
- EF2 = 148sin 130° = 193.20…
- EF = √193.20… = 13.899…
Answer: 13.9 cm
Here is a shape formed from two triangles ABC and CDE. ACD and BCE are straight lines. AC = 24 cm, BC = 31 cm, CE = 19 cm, CD = 16 cm and angle BAC = 64°. Work out the length of DE. Give your answer correct to 3 significant figures.
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- In triangle ABC we know AC, BC and angle A (opposite BC). Use the sine rule to find angle B: sin B = 24 × sin 64°31 = 0.6958…, so B = 44.09…°
- Angle ACB = 180° − 64° − 44.09…° = 71.90…°
- Angle DCE = angle ACB = 71.90…° (vertically opposite angles are equal)
- Cosine rule in triangle CDE: DE2 = 162 + 192 − 2 × 16 × 19 × cos 71.90…° = 256 + 361 − 188.83… = 428.16…
- DE = 20.69…
Answer: 20.7 cm
The diagram shows a prism ABCDEFGH with a horizontal base. The base of the prism, EFGH, is a square of side 12 cm. Trapezium ADEF is a cross section of the prism where AF and DE are vertical edges. DE = CH = 10 cm and AD = BC = 15 cm.
(a) Work out the size of the angle between CF and the base EFGH. Give your answer correct to one decimal place. (3)
(b) Work out the length of BE. Give your answer correct to one decimal place. (3)
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Part (a)
- C is 10 cm vertically above H. The projection of CF onto the base is FH, a diagonal of the square: FH = √122 + 122 = √288 = 16.97… cm
- In right-angled triangle CHF: tan(angle) = CHFH = 1016.97…
- Angle = tan−1(0.5892…) = 30.50…°
Answer: 30.5°
Part (b)
- First find AF. In the trapezium ADEF, AD = 15 is the sloping side, FE = 12 across, so A is √152 − 122 = 9 cm higher than D. AF = 10 + 9 = 19 cm.
- B is directly above G with BG = AF = 19 cm. The base diagonal EG = √288 cm.
- In right-angled triangle BGE: BE2 = 192 + 288 = 361 + 288 = 649
- BE = √649 = 25.47…
Answer: 25.5 cm
Here is a parallelogram PQRS, in which angle SPQ is acute. PQ = 6.1 cm and PS = 3.8 cm. The area of the parallelogram is 18 cm2. Work out the length of QS. Give your answer correct to 3 significant figures.
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- The parallelogram is two identical triangles, so area = PQ × PS × sin P: 6.1 × 3.8 × sin P = 18
- sin P = 1823.18 = 0.7765…, so P = 50.94…° (acute, as stated)
- QS is opposite angle P in triangle PQS. Use the cosine rule: QS2 = 6.12 + 3.82 − 2 × 6.1 × 3.8 × cos 50.94…°
- QS2 = 37.21 + 14.44 − 29.21… = 22.43…
- QS = 4.737…
Answer: 4.74 cm
AB = 4.6 cm, BC = 8.3 cm and angle ABC is acute. The area of triangle ABC is 12 cm2. Work out the perimeter of triangle ABC. Give your answer correct to 3 significant figures.
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- Area = ½ × AB × BC × sin B: ½ × 4.6 × 8.3 × sin B = 12
- sin B = 2438.18 = 0.6286…, so B = 38.94…° (acute)
- Cosine rule: AC2 = 4.62 + 8.32 − 2 × 4.6 × 8.3 × cos 38.94…° = 30.66…
- AC = 5.537… cm
- Perimeter = 4.6 + 8.3 + 5.537… = 18.437…
Answer: 18.4 cm
Here is triangle ABC with AB = 9.7 m, BC = 12.3 m and angle ABC = 115°. Angle ACB = x°. Work out the value of x. Give your answer correct to 3 significant figures.
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- We know two sides and the angle between them, so first find AC with the cosine rule.
- AC2 = 9.72 + 12.32 − 2 × 9.7 × 12.3 × cos 115° = 94.09 + 151.29 + 100.84… = 346.22…
- AC = 18.607… m
- Now use the sine rule. Angle x at C is opposite AB = 9.7: sin x9.7 = sin 115°18.607…
- sin x = 9.7 × sin 115°18.607… = 0.4724…
- x = sin−1(0.4724…) = 28.19…
Answer: x = 28.2
The diagram shows a cube ABCDEFGH with sides of length 6 cm. ABCD is the front face (AB along the top, DC along the bottom), and EFGH is the back face (F behind A, G behind B, H behind C, E behind D). T is the midpoint of AB and V is the midpoint of CH. Work out the distance from T to V in a straight line through the cube. Give your answer in the form √a cm where a is an integer.
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- Use coordinates with D at the origin: across along DC, back along CH, up along DA. Then A = (0, 0, 6), B = (6, 0, 6), C = (6, 0, 0) and H = (6, 6, 0).
- T = midpoint of AB = (3, 0, 6) and V = midpoint of CH = (6, 3, 0)
- Differences: across 6 − 3 = 3, back 3 − 0 = 3, down 6 − 0 = 6
- 3D Pythagoras: TV2 = 32 + 32 + 62 = 9 + 9 + 36 = 54
Answer: √54 cm
The diagram shows a cuboid ABCDEFGH with base EFGH. EH = 9 cm, HG = 5 cm and GB = 6 cm. A is vertically above F. Work out the size of the angle between AH and the plane EFGH. Give your answer correct to 3 significant figures.
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- A is directly above F, so the projection of AH onto the base is FH, and AF = 6 cm.
- FH is a diagonal of the base rectangle (9 by 5): FH = √92 + 52 = √106 = 10.295… cm
- In right-angled triangle AFH: tan(angle) = AFFH = 610.295… = 0.5827…
- Angle = tan−1(0.5827…) = 30.23…°
Answer: 30.2°
Here is quadrilateral ABCD. AB = 15.2 m, AD = 13.4 m, BC = 12.8 m, angle BCD = 62°, angle BDC = 40° and angle DAB = x°. Work out the value of x. Give your answer correct to 3 significant figures.
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- We need angle DAB in triangle ABD. We know AB and AD but not BD, so first find BD from triangle BCD.
- In triangle BCD: BC = 12.8 m is opposite the 40° angle at D, and BD is opposite the 62° angle at C. A side–angle pair is known, so use the sine rule.
- BDsin 62° = 12.8sin 40°, so BD = 12.8 × sin 62°sin 40° = 17.58… m
- Now use the cosine rule in triangle ABD, since all three sides are known: cos x = 13.42 + 15.22 − 17.58…22 × 13.4 × 15.2
- cos x = 179.56 + 231.04 − 309.13…407.36 = 0.2491…
- x = cos−1(0.2491…) = 75.57…
Answer: x = 75.6
In the diagram, AB = 9.4 cm, AD = 12.8 cm, angle BAD = 72°, angle BCD = 54° and angle BDC = 39°. Work out the length of BC. Give your answer correct to 3 significant figures.
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- To reach BC we first need the shared side BD. In triangle ABD we know two sides and the angle between them, so use the cosine rule.
- BD2 = 9.42 + 12.82 − 2 × 9.4 × 12.8 × cos 72° = 88.36 + 163.84 − 74.36… = 177.83…
- BD = 13.335… cm
- In triangle BCD: BD is opposite the 54° angle at C, and BC is opposite the 39° angle at D. Use the sine rule.
- BCsin 39° = BDsin 54°, so BC = 13.335… × sin 39°sin 54° = 10.373…
Answer: 10.4 cm
The diagram shows cuboid ABCDEFGH. AB = 9 cm, AF = 7 cm and the diagonal FC = 18 cm. Calculate the length of BC. Give your answer correct to 3 significant figures.
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- FC is a space diagonal of the cuboid. 3D Pythagoras: FC2 = AB2 + BC2 + AF2 (length, width and height).
- 182 = 92 + BC2 + 72, so 324 = 81 + BC2 + 49
- BC2 = 324 − 130 = 194
- BC = √194 = 13.928…
Answer: 13.9 cm
The diagram shows the prism ABCDEF with cross section triangle ABC (BE is an edge of the prism, perpendicular to the cross section). Angle BEC = 40° and angle ACB is obtuse. AC = 6 cm and CE = 13 cm. The area of triangle ABC is 22 cm2. Calculate the length of AB. Give your answer correct to one decimal place.
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- BE is an edge of the prism, so it is perpendicular to the face ABC and angle CBE = 90°.
- In right-angled triangle CBE: BC is opposite the 40° angle and CE = 13 is the hypotenuse: BC = 13 × sin 40° = 8.356… cm
- Area of triangle ABC = ½ × AC × BC × sin C = 22, so ½ × 6 × 8.356… × sin C = 22
- sin C = 0.8776…, so C = 61.35…° or 180° − 61.35…° = 118.65…°. Angle ACB is obtuse, so C = 118.65…°
- Cosine rule: AB2 = 62 + 8.356…2 − 2 × 6 × 8.356… × cos 118.65…° = 36 + 69.83… + 48.07… = 153.90…
- AB = 12.40…
Answer: 12.4 cm
The diagram shows the prism ABCDEFGHJK with horizontal base AEFG. ABCDE is a cross section of the prism where ABDE is a square and BCD is an equilateral triangle. EF = 2 × AE. M is the midpoint of GF so that JM is vertical. Angle MAJ = y°. Given that tan y° = T, find the value of T, giving your answer in the form √p + √q17 where p and q are integers.
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- Let AE = s. Then EF = 2s, the square has side s and the triangle has side s.
- Height of J above the base = side of square + height of the equilateral triangle = s + √32s = (2 + √3)s2
- M is the midpoint of GF: from A it is s2 across and 2s along. AM = √(s2)2 + (2s)2 = √174s2 = √172s
- tan y° = JMAM = (2 + √3)s2 ÷ √17s2 = 2 + √3√17
- Rationalise: (2 + √3)√1717 = 2√17 + √5117 = √68 + √5117
Answer: T = √68 + √5117
The diagram shows a triangular prism, ABCDEF, with a rectangular base ABCD. AB = 6 cm, DE = 2.2 cm, angle DAE = 18° and angle ADE = 90°. Work out the angle that BE makes with the plane ABCD. Give your answer correct to one decimal place.
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- Triangle ADE (right angle at D): tan 18° = 2.2AD, so AD = 2.2tan 18° = 6.770… cm
- E is directly above D, so the angle we want is angle EBD, in the right-angled triangle EDB.
- BD is the diagonal of the rectangle ABCD: BD2 = 6.770…2 + 62 = 81.84…, so BD = 9.046… cm
- tan EBD = 2.29.046… = 0.2431…, so angle EBD = 13.66…°
Answer: 13.7°
The diagram shows a triangular prism ABCDEF with a horizontal base ABEF. AC = BC = FD = ED = 12 cm, AB = 10 cm and BE = 15 cm. Calculate the size of the angle between AD and the base ABEF. Give your answer correct to 3 significant figures.
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- The angle between a line and a plane is the angle between the line and its projection onto the plane. Drop a perpendicular from D to the base: it lands at M, the midpoint of FE.
- Height DM: triangle FDE is isosceles with FD = 12 and FM = 5, so DM = √122 − 52 = √119 = 10.908… cm
- AM on the base: from A go 5 across (to the middle of the width) and 15 along. AM = √52 + 152 = √250 = 15.811… cm
- In right-angled triangle AMD: tan(angle) = DMAM = 10.908…15.811… = 0.6899…
- Angle = tan−1(0.6899…) = 34.60…°
Answer: 34.6°
The diagram shows a cuboid ABCDEFGH with horizontal base ADEH. AB = 8 cm, AD = 12 cm, DE = 20 cm. M is the midpoint of the base ADEH and P is the midpoint of the edge CF. Work out the size of angle BMP. Give your answer correct to one decimal place.
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- Set up coordinates to find lengths: put A at (0, 0, 0) with AD along the x-axis, DE going back along the y-axis and AB up the z-axis. Then B = (0, 0, 8), C = (12, 0, 8) and F = (12, 20, 8).
- M is the centre of the base, so M = (6, 10, 0). P is the midpoint of CF, so P = (12, 10, 8).
- Use 3D Pythagoras for each side of triangle BMP:
- BM2 = 62 + 102 + 82 = 200
- MP2 = 62 + 02 + 82 = 100
- BP2 = 122 + 102 + 02 = 244
- Cosine rule for the angle at M: cos BMP = BM2 + MP2 − BP22 × BM × MP = 200 + 100 − 2442 × √200 × 10 = 56282.84… = 0.19798…
- Angle BMP = cos−1(0.19798…) = 78.58…°
Answer: 78.6°
Here is a triangle ABC with AB = (2x − 1) cm, AC = (2x + 1) cm and angle BAC = 30°. The area of the triangle is (x2 + x − 3.75) cm2. Find the size of the largest angle in triangle ABC. Give your answer correct to the nearest degree.
Show solution
- Area = ½ × AB × AC × sin 30° = ½(2x − 1)(2x + 1) × ½ = ¼(4x2 − 1) = x2 − 0.25
- Set equal to the given area: x2 − 0.25 = x2 + x − 3.75, so x = 3.5
- So AB = 6 cm and AC = 8 cm.
- Find BC with the cosine rule: BC2 = 62 + 82 − 2 × 6 × 8 × cos 30° = 100 − 83.138… = 16.861…, so BC = 4.106… cm
- The largest angle is opposite the longest side, AC = 8 cm, so it is angle B.
- Cosine rule: cos B = 62 + 4.106…2 − 822 × 6 × 4.106… = −11.138…49.275… = −0.2260…
- B = cos−1(−0.2260…) = 103.06…°
Answer: 103°
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