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The diagram shows a triangle ABC, AB is the shortest side - AQA - A-Level Maths Pure - Question 4 - 2022 - Paper 2

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The diagram shows a triangle ABC, AB is the shortest side. The lengths of AC and BC are 6.1 cm and 8.7 cm respectively. The size of angle ABC is 38° Find the size... show full transcript

Worked Solution & Example Answer:The diagram shows a triangle ABC, AB is the shortest side - AQA - A-Level Maths Pure - Question 4 - 2022 - Paper 2

Step 1

Use the Sine Rule

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Answer

To find the largest angle in triangle ABC, use the Sine Rule, which states: asinA=bsinB=csinC\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}

Let:

  • AC = 6.1 cm,
  • BC = 8.7 cm,
  • angle ABC = 38°.

We can denote:

  • AB = c,
  • angle A = C,
  • angle B = ABC, and
  • angle A = 180° - A - B.

Step 2

Substituting Values

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Answer

Substituting the known values into the sin rule yields:

csin(38°)=6.1sin(A)=8.7sin(180°A38°)\frac{c}{\sin(38°)} = \frac{6.1}{\sin(A)} = \frac{8.7}{\sin(180° - A - 38°)}

Now, let’s find the length of side AB (c).

Calculating: c=8.7sin(38°)sin(A)c = \frac{8.7 \cdot \sin(38°)}{\sin(A)}

Step 3

Finding Angle A

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Answer

Using the Sine Rule again:

Calculating angle A gives: A=arcsin(6.1sin(38°)8.7)61° (rounded to the nearest degree)A = \arcsin\left(\frac{6.1 \cdot \sin(38°)}{8.7}\right) \approx 61° \text{ (rounded to the nearest degree)}

Now, since angle A + angle B + angle C = 180°, we can identify the largest angle.

Step 4

Determine the Largest Angle

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Answer

Calculating angle C: C=180°A38°180°61°38°81°C = 180° - A - 38° \approx 180° - 61° - 38° \approx 81°

Thus, the largest angle in triangle ABC is approximately:

119°119°

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