In Δ ABC, ∠B = 135°, AB = 10√2 cm and BC = 14 cm. What is the length of AC? 

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AAI Junior Assistant (Fire Service) Official Paper (Held On: 15 Nov 2022 Shift 1)
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  1. 20√2 cm
  2. 28 cm
  3. 26 cm
  4. 24√2 cm

Answer (Detailed Solution Below)

Option 3 : 26 cm
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Detailed Solution

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Given:

In Δ ABC, ∠B = 135°, AB = 10√2 cm and BC = 14 cm.

Formula used:

Using the Cosine Rule:

\(c^2 = a^2 + b^2 - 2ab \cos(C)\)

Calculations:

5-5-2025 IMG-1083 Prakhar -5

Let AB = c = 10√2 cm, BC = a = 14 cm, and AC = b.

∠B = 135°

Using the Cosine Rule:

\(b^2 = a^2 + c^2 - 2ac \cos(B)\)

\(b^2 = 14^2 + (10√2)^2 - 2 \times 14 \times 10√2 \times \cos(135°)\)

⇒ \(b^2 = 196 + 200 - 2 \times 14 \times 10√2 \times -\frac{1}{√2}\)

⇒ \(b^2 = 196 + 200 + 2 \times 14 \times 10\)

⇒ \(b^2 = 676\)

⇒ \(b = 26\) cm

∴ The correct answer is option (3).

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