# If in an inner product space $\langle x,u\rangle=\langle x,v\rangle$ for all $x$, show that $u=v$.

If in an inner product space $\langle x,u\rangle=\langle x,v\rangle$ for all $x$, show that $u=v$.

This seems obvious to me, so how do I prove it? Proof by contradiction maybe?

Any suggestions would be nice.

• Please don't use $<$ and $>$ for inner products. Those are relation symbols, and they not only look wrong, they also get the spacing associated with relations. Use \langle and \rangle to get $\langle$ and $\rangle$. Oct 7 '14 at 14:02

Let $x=u-v$, then:
$$\langle x,u\rangle-\langle x,v\rangle=\langle x,u-v\rangle=\langle u-v,u-v\rangle=0$$
So $u-v=0$.