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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 the maximum area of the triangle?

Options

  1. A32\dfrac{\sqrt{3}}{2} square unit
  2. B3\sqrt{3} square units
  3. C62\dfrac{\sqrt{6}}{2} square units
  4. D6\sqrt{6} square units

Correct answer

A. 32\dfrac{\sqrt{3}}{2} square unit

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 a2+c2=b2a^2 + c^2 = b^2.

We are given AB+AC=3AB + AC = 3, which means c+b=3⇒b=3−cc + b = 3 \Rightarrow b = 3 - c.

Substituting bb into the Pythagorean theorem:

a2+c2=(3−c)2a^2 + c^2 = (3 - c)^2

a2+c2=9−6c+c2a^2 + c^2 = 9 - 6c + c^2

a2=9−6c⇒a=9−6ca^2 = 9 - 6c \Rightarrow a = \sqrt{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=Δ2S = \Delta^2:

S=14c2(9−6c)=94c2−32c3S = \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:

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

92c(1−c)=0\dfrac{9}{2}c(1 - c) = 0

Since c>0c > 0, we get c=1c = 1.

For c=1c = 1, d2Sdc2=92−9c=−92<0\dfrac{d^2S}{dc^2} = \dfrac{9}{2} - 9c = -\dfrac{9}{2} < 0, which confirms a maximum.

Substituting c=1c = 1 into the expression for area:

Δ=12(1)9−6(1)=32\Delta = \dfrac{1}{2}(1)\sqrt{9 - 6(1)} = \dfrac{\sqrt{3}}{2} square units.

Answer: 32\dfrac{\sqrt{3}}{2} square unit
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