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WE KNOW THAT sec^2(a)>=1

hence 4xy/(x+y)^2>=1

hence,4xy>=(x+y)^2

which gives (x-y)^2<=0

thus only equality will hold since a square term can't be zero

hence (x-y)=0 i.e.x=y

and x or y can't be zero because sec^2(a) can't be zero