Cos A = x^2 - y^2 / x^2 + y^2 where x € r and y € r gives real number A if and only if ---
a) x= y not equal 0
b) l x l = l y l not equal 0
c) x + y = 0 , x not equal 0
d) none of these pls give chose the correct option .with explanation .
pls give the answer fast .
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