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1 Answer
P dilip_k
Aug 19, 2017

#x^2+y^2#

#=(acostheta -bsintheta)^2+(bcostheta +asintheta)^2#

#=a^2cos^2theta +b^2sin^2theta-2abcosthetasintheta+b^2cos*2theta +a^2sin^2theta+2abcosthetasintheta#

#=a^2(cos^2theta +sin^2theta)+b^2(cos*2theta +sin^2theta)#

#=a^2+b^2#

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