Symmetric bilinear form/Positivity on orthogonal basis/Exercise

Let $V$ be a finite-dimensional real vector space, endowed with a symmetric bilinear form $⟨-,-⟩$ on $V$. Let $u_1,\dots ,u_n$ be an orthogonal basis of $V$ with the property $⟨u_i,u_i⟩>0$ for all $i=1,\dots ,n$. Show that $⟨-,-⟩$ is positive definite ist.

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