
I've thought about this question for about an hour but I'm still not able to arrive at correct answer. Can anyone suggest me how to go about it?

I've thought about this question for about an hour but I'm still not able to arrive at correct answer. Can anyone suggest me how to go about it?
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We have, by definition of the adjoint $\def\<#1>{\left<#1\right>}$ $$ y^tBCx = \<S(x), y> = \<x,T(y)> = \<x,Ay> = (Ay)^tBx = y^tA^tBx$$ As this holds for all $x, y \in \mathbf R^n$, we must have $BC = A^tB$.