# Math Help - Addition/Subtraction of Cosine

If I am using the addition and subtraction formula, how would I simplify cos(x + [pi])?

2. Originally Posted by mike1
If I am using the addition and subtraction formula, how would I simplify cos(x + [pi])?
cos(A + B) = cos(A)cos(B) - sin(A)sin(B)

So
cos(x + [pi]) = cos(x)cos([pi]) - sin(x)sin([pi])

cos([pi]) = -1
sin([pi]) = 0

Thus
cos(x + [pi]) = -cos(x)

-Dan

3. That helps, thanks alot.