- #1

happyparticle

- 400

- 20

**Mentor note**: Moved from a technical section, so is missing the homework template.

Hi,

I'm always not sure how to prove something in math and I'm wondering if this is enough.

##\vec r \cdot (\vec u + \vec v) ##

##\vec u + \vec v = (u_1+v_1, u_2+v_2,u_3+v_3) = \vec s##

##\vec r \cdot (\vec u + \vec v) = \vec r \cdot \vec s ##

## \sum_{i=1}^3 r_is_i = r_1s_1 + r_2s_2 + r_3s_3 = \vec r \cdot \vec s = \vec r \cdot (\vec u + \vec v)##

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