A useful identity for those whose calculators lack an easy way to do products, but can use summation.

Note that this method works best with

**Theorem:**

**Proof:**

Equal by definition.

∎

And as a direct result (take the of both sides):

A useful identity for those whose calculators lack an easy way to do products, but can use summation.

Note that this method works best with

**Theorem:**

**Proof:**

Equal by definition.

∎

And as a direct result (take the of both sides):