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Application Of Derivatives

Question
CBSEENMA12035528

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

Solution
Let x be the side of the square and r be the radius of the circle. Let P be the sum of perimeters of square and circle.
             therefore space space space space space space space space space straight P space equals space 4 straight x plus 2 πr                                              ...(1)
Let A be the sum of areas of squares and circle
therefore space space space space space space space space space space space space space space space space space space space straight A space equals space straight x squared plus πr squared
or space space space space space space space space space space space space space space space space space space space space straight A space equals space open parentheses fraction numerator straight P minus 2 πr over denominator 4 end fraction close parentheses squared space plus space πr squared space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space open square brackets because space of space left parenthesis 1 right parenthesis close square brackets
therefore space space space space space space space space space space space space space space space dA over dr space equals space 2 open parentheses fraction numerator straight P minus 2 πr over denominator 4 end fraction close parentheses. space space open parentheses negative fraction numerator 2 straight pi over denominator 4 end fraction close parentheses space plus space 2 space πr
space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space minus straight pi over 4 left parenthesis straight P minus 2 πr right parenthesis space plus space 2 πr
 For A to be minimum or minimum,
                            space space space space space space space space space space space space space space space space space dA over dr space equals space 0 space space space space space space space space space space space space space space space space space space space space space space space rightwards double arrow space space minus straight pi over 4 left parenthesis straight P minus 2 straight pi right parenthesis space plus space 2 space πr space equals space 0
rightwards double arrow space space space space space space space space space space space space space space minus straight pi left parenthesis straight P minus 2 πr right parenthesis space plus space 8 space πr space equals 0 space space space space rightwards double arrow space space space space space minus πP plus 2 straight pi squared straight r space plus space 8 space straight pi space straight r space equals space 0
rightwards double arrow space space space space space space space space space space space space space space space space space space space space left parenthesis 2 straight pi squared plus 8 straight pi right parenthesis space straight r space equals space πP space space space space space space space space space rightwards double arrow space space 2 left parenthesis straight pi plus 4 right parenthesis space straight r space equals space straight P space space space space
rightwards double arrow space space space space space space space space space space space space space space space space space space space space space space space straight r space equals space fraction numerator straight P over denominator 2 left parenthesis straight pi plus 4 right parenthesis end fraction
                       fraction numerator straight d squared straight A over denominator dr squared end fraction space equals space minus straight pi over 4 left parenthesis 0 minus 2 straight pi right parenthesis space plus space 2 space straight pi space equals space straight pi squared over 2 plus 2 straight pi
When space space space space space space space space space space space straight r space equals space fraction numerator straight P over denominator 2 left parenthesis straight pi plus 4 right parenthesis end fraction comma space space space fraction numerator straight d squared straight A over denominator dr squared end fraction space equals space straight pi squared over 2 plus 2 straight pi greater than 0
therefore space space space space space space straight A space is space minimum space when space space space space space space space space space space space space space space space space space space space
                                   straight r space equals space fraction numerator straight P over denominator 2 left parenthesis straight pi plus 4 right parenthesis end fraction space space space space space space space space space space space space space space space space space space space space space space space space space space space space straight i. straight e. space space space straight r space equals fraction numerator 4 straight x plus 2 πr over denominator 2 left parenthesis straight pi plus 4 right parenthesis end fraction space space space space space space space space open square brackets because space of space left parenthesis 1 right parenthesis close square brackets space
straight i. straight e. space space space space space space space straight r space equals space fraction numerator 2 straight x plus πr over denominator straight pi plus 4 end fraction space space space space space space or space space space space πr plus 4 straight r space equals space 2 straight x plus πr
straight i. straight e. space space space space space space 4 straight r space equals space 2 straight x space space space space space space space space space straight i. straight e. comma space space space space straight x space equals space 2 straight r
i.e.  side of the square  = diameter of circle.