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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Application of Derivatives

NDA 2025 · Mathematics · Application of Derivatives

Consider the following for the two (02) items that follow: Let ABC...

Consider the following for the two (02) items that follow:
Let ABCABC be a triangle right-angled at BB and AB+AC=3AB + AC = 3 units.

What is ∠A\angle A equal to if the area of the triangle is maximum?

Options

  1. Aπ6\dfrac{\pi}{6}
  2. Bπ4\dfrac{\pi}{4}
  3. Cπ3\dfrac{\pi}{3}
  4. D5π12\dfrac{5\pi}{12}

Correct answer

C. π3\dfrac{\pi}{3}

Step-by-step solution

Let AB=cAB = c, BC=aBC = a, and AC=bAC = b.

Given that the triangle is right-angled at BB, we have b2=a2+c2b^2 = a^2 + c^2.

We are also given AB+AC=3⇒c+b=3⇒b=3−cAB + AC = 3 \Rightarrow c + b = 3 \Rightarrow b = 3 - c.

Substituting bb in the Pythagoras theorem:

a2=(3−c)2−c2=9−6c+c2−c2=9−6ca^2 = (3 - c)^2 - c^2 = 9 - 6c + c^2 - c^2 = 9 - 6c

The area of the triangle is Δ=12ac=12c9−6c\Delta = \dfrac{1}{2}ac = \dfrac{1}{2}c\sqrt{9 - 6c}.

To maximize the area, we can maximize its square:

S=Δ2=14c2(9−6c)=94c2−32c3S = \Delta^2 = \dfrac{1}{4}c^2(9 - 6c) = \dfrac{9}{4}c^2 - \dfrac{3}{2}c^3

Differentiating SS with respect to cc and equating to zero for maximum area:

dSdc=92c−92c2=0\dfrac{dS}{dc} = \dfrac{9}{2}c - \dfrac{9}{2}c^2 = 0

c(1−c)=0⇒c=1c(1 - c) = 0 \Rightarrow c = 1 (since c>0c > 0)

For c=1c = 1, b=3−1=2b = 3 - 1 = 2.

In the right-angled △ABC\triangle ABC, we have:

cos⁡A=ABAC=cb=12\cos A = \dfrac{AB}{AC} = \dfrac{c}{b} = \dfrac{1}{2}

Therefore, ∠A=π3\angle A = \dfrac{\pi}{3}.

Answer: π3\dfrac{\pi}{3}
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