# Math Help - cos (x - y)

1. ## cos (x - y)

When sin x = -8/17 and lies in quadrant 3 and cos y = -4/5 and lies in quadrant 2, find cos (x - y).

2. Originally Posted by magentarita
When sin x = -8/17 and lies in quadrant 3 and cos y = -4/5 and lies in quadrant 2, find cos (x - y).
$cos(x-y) = cos(x)cos(y) + sin(x)sin(y)$

3. ## Tell me.........

Originally Posted by Mush
$cos(x-y) = cos(x)cos(y) + sin(x)sin(y)$
Tell me, what is the name of that formula?

4. ## Re:

That is one of the additional formulas .

5. ## ok...

Originally Posted by mathaddict
That is one of the additional formulas .
I understand. Thanks.