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Given the sum of the perimeters of a square and a circle, show that the sum of their areas is least when one side of the square is equal to diameter of the circle.

Let us say the sum of perimeter of square and circumference of circle be $L$

Given sum of the perimeters of a square and a circle.

Assuming, \[side\text{ }of\text{ }square\text{ }=\text{ }a\text{ }and\text{ }radius\text{ }of\text{ }circle\text{ }=\text{ }r\]

Then, \[L\text{ }=\text{ }4a\text{ }+\text{ }2\pi r\text{ }\Rightarrow \text{ }a\text{ }=\text{ }\left( L\text{}-\text{}2\pi r \right)/4\ldots \text{ }\left(1\right)\]

Let the sum of area of square and circle be $S$

So, \[S\text{ }=\text{ }{{a}^{2}}~+\text{ }\pi {{r}^{2}}\]

RD Sharma Solutions for Class 12 Maths Chapter 18 Maxima and Minima Image 34