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**AlexP** I'm trying to prove Parseval's Identity.

"Let $\displaystyle \{v_1, ..., v_n\}$ be an orthonormal basis for $\displaystyle V$. For any $\displaystyle x, y \in V$ prove that $\displaystyle \langle x,y \rangle = \displaystyle\sum^n_{i=1} \langle x,v_i \rangle \overline{\langle y,v_i \rangle}$."

I'm really not sure how to go about this. I've played around with the summand, including taking $\displaystyle x$ as a linear combination of the $\displaystyle v_i$ and breaking it down, but that actually just gets us right back to $\displaystyle \langle x,v_i \rangle$.

I'd like some hints, but only hints. Thanks.

Just do what's natural. By definition you have that $\displaystyle x=\langle x,v_1\rangle v_1+\cdots+\langle x,v_n\rangle v_n$ and $\displaystyle y=\langle y,v_1\rangle y_1+\cdots+\langle y,v_n\rangle v_n$ and so

$\displaystyle \displaystyle \langle x,y\rangle=\left\langle \sum_{i=1}^{n}\langle x,v_i\rangle v_i,\sum_{j=1}^{n}\langle y,v_i\rangle v_i\right\rangle$

so what if we expand by sequilinearity?