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If for every $v\in V$ $\langle v,v\rangle_{1} = \langle v,v \rangle_{2}$ then $\langle\cdot,\cdot \rangle_{1} = \langle\cdot,\cdot \rangle_{2}$
Let V be a real vector space, and let $\langle x,y \rangle_1$ and $\langle x,y \rangle_2$ be two inner products defined on V.
How would you prove that if $\langle x,x \rangle_1$ = $\langle x,x \rangle_2$ $\forall x\in V$ then $\langle x,y \rangle_1$ = $\langle x,y \rangle_2$ $\forall x,y\in V$?