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

NDA 2025 · Mathematics · Application of Derivatives

A wire of length 20 cm is to be bent into a rectangle. Which of the...

A wire of length 20 cm is to be bent into a rectangle. Which of the following statements is/are correct?
I. The rectangle of the largest area is the square.
II. It is possible to form a rectangle of an area of 27 cm^2.
Select the answer using the code given below.

Options

  1. AI only
  2. BII only
  3. CBoth I and II
  4. DNeither I nor II

Correct answer

A. I only

Step-by-step solution

Let the length and breadth of the rectangle be xx and yy respectively.

Given the perimeter of the rectangle is 20 cm20\text{ cm}.

2(x+y)=20⇒x+y=102(x + y) = 20 \Rightarrow x + y = 10

The area of the rectangle is A=xyA = xy.

Substituting y=10−xy = 10 - x, we get:

A=x(10−x)=10x−x2A = x(10 - x) = 10x - x^2

To find the maximum area, we can complete the square:

A=25−(x−5)2A = 25 - (x - 5)^2

The maximum value of AA is 25 cm225\text{ cm}^2, which occurs when x=5 cmx = 5\text{ cm}.

When x=5 cmx = 5\text{ cm}, y=10−5=5 cmy = 10 - 5 = 5\text{ cm}.

Since x=yx = y, the rectangle of the largest area is a square. Thus, statement I is correct.

The maximum possible area is 25 cm225\text{ cm}^2. It is not possible to form a rectangle of area 27 cm227\text{ cm}^2. Thus, statement II is incorrect.

Only statement I is correct.

Answer: I only
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