Printable · GCSE Higher · ages 14-16
Area of a triangle: ½ab sin C worksheet — GCSE Higher
Fifteen questions on "area of a triangle: ½ab sin c" — DfE statement G23. Print it, or print three versions so neighbours cannot copy by letter; the key gives the letter for each version.
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Answer key: Area of a triangle: ½ab sin C worksheet — GCSE Higher
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- (d) 114.6° — Method: rearrange Area = (1/2)ab sin C to make sin C the subject, then use the obtuse branch since the question states the angle is obtuse. Working: sin C = 2 × 45 ÷ (11 × 9) = 0.909, so the acute angle is sin⁻¹(0.909) = 65.4°, and the obtuse angle is 180 − 65.4 = 114.6°. Answer: 114.6°. Giving the acute angle straight from the calculator, 65.4°, ignores that the question asks for the obtuse one; forgetting to double the area before dividing gives sin C = 45 ÷ (11 × 9) = 0.4545, whose obtuse angle is 180 − 27.0 = 153.0°; and that same forgotten-doubling error taken on the acute branch instead gives 27.0°. Always double the area first, and then take 180° minus the calculator's answer whenever the question specifically asks for the obtuse angle.
- (a) 3 — Method: find the sail's area with Area = (1/2)ab sin C, then divide by how much one litre covers, and round up to a whole number of tins since paint can only be bought by the tin. Working: Area = 1/2 × 4.2 × 3.6 × sin 115° = 6.85 m² (2 d.p.); 6.85 ÷ 3 = 2.28 tins, which rounds up to 3 tins. Answer: 3. Rounding 2.28 to the nearest whole number instead of rounding up gives 2, which is not enough paint to finish the sail; forgetting the 1/2 in the area formula gives an area of 13.7 m², needing 5 tins; and assuming one litre covers exactly 1 m² of sail, ignoring the coverage rate given, rounds the area itself up to 7 tins. Whenever a quantity can only be bought in whole units, round up even when the decimal part is small — 2.28 tins means you need a third tin.
- (c) 49.5 m² — Method: the area formula needs two sides and the angle between them, and only sides are given, so find one angle with the cosine rule first and then use the two sides that enclose it. Working: angle ABC lies between AB = 14 m and BC = 9 m, so cos ABC = (14² + 9² − 11²) ÷ (2 × 14 × 9) = (196 + 81 − 121) ÷ 252 = 156 ÷ 252 = 0.61905, giving angle ABC = 51.753° and sin ABC = 0.78535. The area is then 1/2 × 14 × 9 × 0.78535 = 63 × 0.78535 = 49.477. Answer: the area is 49.5 m² to 1 decimal place. The distractors: 60.5 m² comes from using the correct angle at B with the sides 14 m and 11 m, which do not both meet at B, so the angle is no longer the one they enclose; 63.0 m² comes from leaving the sine out and treating the two sides as a base and a perpendicular height; 40.5 m² comes from working out angle BAC = 39.98° instead and using its sine with the sides that meet at B, so that the angle used is not the angle between them.
- (c) $12\sqrt{3}$ cm² — Use Area = 1/2 × AB × AC × sin(angle BAC) = 1/2 × 6 × 8 × sin 60°. Since sin 60° = √3⁄2, this is 1/2 × 6 × 8 × √3⁄2 = 24 × √3⁄2 = 12√3 cm². Using AB × AB instead of AB × AC (taking 6 × 6 rather than 6 × 8) gives 1/2 × 6 × 6 × sin 60° = 18 × √3⁄2 = 9√3 cm². Using AC × AC instead of AB × AC (taking 8 × 8 rather than 6 × 8) gives 1/2 × 8 × 8 × sin 60° = 32 × √3⁄2 = 16√3 cm². Leaving out the 1/2 from the area formula gives 6 × 8 × sin 60° = 48 × √3⁄2 = 24√3 cm².
- (c) 152.2 cm² — Method: find the area of one of the 5 isosceles triangles with 1/2ab sin C, then multiply by 5 for the whole pentagon. Working: one triangle has area 1/2 × 8 × 8 × sin 72° = 30.4338 cm², so the pentagon's area = 5 × 30.4338 = 152.2 cm² — keep the unrounded triangle area, since 5 × 30.4 would give 152.0. Reporting the area of a single triangle and forgetting to multiply by 5 gives 30.4 cm²; multiplying by 6 instead of 5, as for a hexagon, gives 182.6 cm²; and leaving out the 1/2 from each triangle before multiplying by 5 gives 304.3 cm². A regular pentagon splits into exactly 5 triangles at its centre, each with a 72° angle, since 360° ÷ 5 = 72°.
- (b) £709 — Method: first find the area of the triangular platform with Area = (1/2)ab sin C, then multiply by the cost per square metre. Working: Area = 1/2 × 11.5 × 8.2 × sin 72° = 44.8 m² (1 d.p.); cost = area × £15.80, which rounds to £709 to the nearest pound. Answer: £709. Leaving out the 1/2 in the area formula doubles the area, giving a cost of £1417; using cos 72° instead of sin 72° gives a much smaller area and a cost of £230; and squaring the 11.5 m side instead of multiplying the two different given sides together gives a cost of £994. Find the exact area first — don't round it early — then multiply by the cost per square metre and round only the final answer.
- (b) 46.8 cm² — Method: for a parallelogram, not a triangle, the two sides and the angle between them give Area = ab sin C — there is no 1/2. Working: Area = 6 × 9 × sin 60° = 46.8 cm² (1 d.p.). Answer: 46.8 cm². Using the triangle formula, (1/2)ab sin C, on a parallelogram by mistake gives half the true area, 23.4 cm²; using cos 60° instead of sin 60° gives 27.0 cm²; and adding the two sides before multiplying by sin 60° gives 13.0 cm². A parallelogram is exactly two of the triangles this formula was built for, so never carry the 1/2 across from the triangle version.
- (d) Triangle 2, by 3.6 cm² — Method: find both areas with 1/2ab sin C, then compare them. Working: area of triangle 1 = 1/2 × 10 × 13 × sin 64° = 58.4 cm²; area of triangle 2 = 1/2 × 11 × 12 × sin 70° = 62.0 cm²; triangle 2 is larger, by 62.0 − 58.4 = 3.6 cm². Getting the right difference but naming triangle 1 as the larger one, the subtraction done the wrong way round, gives 'Triangle 1, by 3.6 cm²'; leaving out the 1/2 when finding triangle 1's area (giving 116.8 cm² instead of 58.4 cm²) and then subtracting gives 'Triangle 1, by 54.8 cm²'; and using cos 64° instead of sin 64° for triangle 1 (giving 28.5 cm² instead of 58.4 cm²) gives 'Triangle 2, by 33.5 cm²'. Work out both areas fully and correctly before comparing which is bigger.
- (d) 30.2 cm² — Method: with two sides and the angle between them, use Area = (1/2)ab sin C. Working: Area = 1/2 × 9.4 × 7.2 × sin 63° = 30.2 cm² (1 d.p.). Answer: 30.2 cm². Leaving out the 1/2 altogether gives 60.3 cm²; using cos 63° instead of sin 63° gives 15.4 cm²; and squaring one side instead of multiplying the two different given sides together gives 39.4 cm². Always check you are using sin, not cos, and that the 1/2 is there before you multiply the two given sides together.
- (c) 62.9 cm² — Method: no two sides are given, so first find AC with the sine rule, then find the area using BC, AC and the angle between them, angle ACB. Working: angle BAC = 180° − 58° − 47° = 75°; by the sine rule, AC = 14 × sin 58° / sin 75° = 12.2915 cm; then area = 1/2 × 14 × 12.2915 × sin 47° = 62.9 cm² — keep the unrounded AC, since rounding it to 12.3 cm shifts the area to 63.0 cm². Pairing 14 with sin 75° and dividing by sin 58° instead (the ratio the wrong way round) gives AC = 15.9 cm and an area of 81.6 cm²; using angle BAC = 75° as the included angle instead of angle ACB gives 83.1 cm²; and assuming the triangle is isosceles with AC = BC = 14 cm, skipping the sine rule step entirely, gives 71.7 cm². The angle used in the area formula must be the one between the two sides being multiplied — here that is angle ACB, between BC and AC.
- (d) 53.2 cm² — Method: diagonal AC splits the kite into two congruent triangles, ABC and ADC, each with area 1/2 × AB × CB × sin(ABC), so the whole kite has area 2 × 1/2 × AB × CB × sin(ABC) = AB × CB × sin(ABC). Working: kite area = 6 × 9 × sin 100° = 53.2 cm². Reporting just one triangle's area, 1/2 × 6 × 9 × sin 100°, and forgetting to double it for the whole kite gives 26.6 cm²; multiplying the two sides together without any sine term at all gives 54.0 cm²; and doubling the triangle area twice, as if the kite were made of four congruent triangles instead of two, gives 106.4 cm². A kite split by its axis of symmetry always gives exactly two congruent triangles.
- (d) 118.0 cm² — Method: a rhombus is made of two congruent triangles either side of a diagonal, each with area 1/2 × 12 × 12 × sin 55°, so the whole rhombus has area 12 × 12 × sin 55° (side² × sin of the interior angle). Working: area = 12² × sin 55° = 118.0 cm². Stopping at one triangle's area, 1/2 × 12² × sin 55°, and forgetting to double it gives 59.0 cm²; using cos 55° instead of sin 55° gives 82.6 cm²; and multiplying the two sides together with no trig term at all gives 144.0 cm². Splitting the rhombus into its two triangles is the safest way to see why the 1/2 disappears from the whole-shape formula.
- (a) 10.1 cm — Method: the area formula 1/2 × a × b × sin C contains the unknown side, so substitute what is known and rearrange. Working: 42 = 1/2 × 9.5 × AC × sin 61°. Multiplying both sides by 2 gives 84 = 9.5 × AC × sin 61°, and 9.5 × sin 61° = 9.5 × 0.87462 = 8.3089, so AC = 84 ÷ 8.3089 = 10.1096. Answer: AC = 10.1 cm to 1 decimal place. The distractors: 5.1 cm comes from forgetting to double the area when clearing the factor 1/2 and working out 42 ÷ 8.3089; 18.2 cm comes from using cos 61° in place of sin 61° in the denominator; 7.7 cm comes from multiplying by sin 61° instead of dividing by it, 84 × sin 61° ÷ 9.5, the standard slip when the unknown is inside a product.
- (d) £817 — Method: find the area of the plot with 1/2 × a × b × sin C, then multiply the area by the cost of a square metre. Working: the 108° angle is between AB and AC, so the area is 1/2 × 23.5 × 17.2 × sin 108° = 202.1 × 0.95106 = 192.21 m². The cost is 192.21 × 4.25 = 816.89. Answer: the turf costs £817 to the nearest pound. The distractors: £1634 comes from leaving out the factor 1/2, so the area is taken as 384.42 m²; £859 comes from leaving the sine out and using 202.1 m² as the area, which treats the two sides as a base and a perpendicular height; £192 is the area of the plot written down as though it were the cost, stopping one step short of the question.
- (c) No — 50° is angle P, not between PQ and QR. — Method: 1/2ab sin C only gives the area when C is the angle between the two named sides. Working: PQ and QR meet at vertex Q, so the angle between them is angle PQR, not angle QPR (which is the angle at vertex P, between sides PQ and PR). Since angle QPR is not included between PQ and QR, the student's calculation 1/2 × 8 × 11 × sin 50° does not give the area of the triangle — the verdict is No. Claiming that any angle works with any two sides ignores that the formula is only true for the INCLUDED angle. Claiming the formula needs all three sides is a different, incorrect statement about the formula itself — Area = 1/2ab sin C is valid with exactly two sides and their included angle, when that angle is actually known.
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