# Finding cos(a-B) using tan a = 4/3 in quad III and cos B = 12/13 in quad IV

Really stuck with this problem, I need help with this asap.

To find cos(a-B), I'm supposed to use:

tan a = 4/3, with a in quadrant III

cos B = 12/13, with B in quadrant IV

• Do you know that $cos(A-B)=cosAcosB+sinAsinB$? – Indrayudh Roy May 13 '14 at 10:28
• Of course, I started by using that formula, but I'm still having trouble. – user139473 May 13 '14 at 10:48