- #1

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## Homework Statement

The following are all vectors:

a, b, c, d

k = (a+b) (c+d)

prove: k = (a x c) + (a x d) + (b x c) + (b x d)

## Homework Equations

## The Attempt at a Solution

I have tried to start it by doing:

|a| = a = sqrt(a_x^2 + a_y^2)

is that a correct start at it? I really am pretty lost...