- #1

ER901

- 2

- 0

## Homework Statement

I need to prove the following equation:

cos^2 A + cos^2 B + cos^2 C + 2*cosA*cosB*cosC = 1

where A,B,C are the angles of a triangle

## Homework Equations

A+B+C = 180

## The Attempt at a Solution

I substituted the sum of A,B,C into the equation, and now I have something like

... + cos^2(180-A-C)+... so on, but I don't know what to do next.

Thanks