Question

Sin(a)=7/25, a lies in quadrant I, and cos(b)=8/17, b lies in quadrant I. Find cos(a+b)

Answer 1

I think that you will need this formula to get started.

Answer 2

Yes I have used that formula. The only problem I'm having is that when I use (8/17)^2+sin^2b=1 to find sin^2b, I end up with 15/17. I then input this into the formula: (24/25)(8/17)-(7/25)(15/17) and I end up with 87/425 which does't equate cos(24/25+8/17). What am I doing wrong?

Answer 3

A and B are angles. You appear to be entering sines or cosines of the angles.