How to get scalar product from geometric product in Geometric Algebra?

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I read that geometric product $ab$ can be decomposed into 2 parts (symmetric and antisymmetric).

But I can't understand why symmetric part is a scalar product.

I mean following symmetric part:

$\frac{1}{2} (ab+ba) = \frac{1}{2}((a+b)^2-a^2-b^2)$

Why $ab+ba = (a+b)^2-a^2-b^2$ and $\frac{1}{2} (ab+ba) = a \cdot b $ ?

Thanks.