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

Question

directrix · Answer

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

anonymous · Answer

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?

directrix · Answer

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

directrix · Answer

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