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CBSE Class 12-science Answered

Show that the maximum area of the rectangle that can be inscribed in circle of radius r is 2r^2   
Asked by ramanujkumargond450 | 01 Sep, 2020, 12:19: PM
answered-by-expert Expert Answer
Let space ABCD space be space the space rectangle space inscribed space in space straight a space circle space of space radius space straight r.
AC space and space BD thin space are space diameters space of space length space 2 straight r space as space angle space in space semicircle space is space always space 90 degree.
Let space straight x space be space length comma space straight y space be space breadth space of space rectangle.
straight x squared plus straight y squared equals left parenthesis 2 straight r right parenthesis squared
rightwards double arrow straight y squared equals 4 straight r squared minus straight x squared
rightwards double arrow straight y equals square root of 4 straight r squared minus straight x squared end root
Area space of space reactangle comma space straight A equals xy
straight A equals straight x square root of 4 straight r squared minus straight x squared end root
rightwards double arrow dA over dx equals square root of 4 straight r squared minus straight x squared end root minus fraction numerator 2 straight x squared over denominator 2 square root of 4 straight r squared minus straight x squared end root end fraction
rightwards double arrow dA over dx equals fraction numerator 4 straight r squared minus straight x squared minus straight x squared over denominator square root of 4 straight r squared minus straight x squared end root end fraction
rightwards double arrow dA over dx equals fraction numerator 4 straight r squared minus 2 straight x squared over denominator square root of 4 straight r squared minus straight x squared end root end fraction
For space maximum space area comma
dA over dx equals 0
rightwards double arrow fraction numerator 4 straight r squared minus 2 straight x squared over denominator square root of 4 straight r squared minus straight x squared end root end fraction equals 0
rightwards double arrow 4 straight r squared minus 2 straight x squared equals 0
rightwards double arrow straight x equals square root of 2 straight r
Also comma space straight y equals straight r square root of 2
Area space of space the space rectangle space equals space length space cross times space breadth space equals space 2 straight r squared.
Answered by Renu Varma | 01 Sep, 2020, 02:21: PM
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