Question 824857
Given that Sin A = 4/5, 0 < A < &#960;/2 and Cos B = -12/13, &#960;< B < 3&#960;/2, find Cos (A - B)
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Identity: cos(A-B)=cosAcosB+sinAsinB
sinA=4/5 (working with (3-4-5) reference right triangle in quadrant I in which sin and cos>0.
cosA=3/5
..
cosB=-12/13 (working with (5-12-13) reference right triangle in quadrant III in which sin and cos<0.
sinB=-5/13
..
{{{cos(A-B)=((3/5)*(-12/13))+((4/5)*(-5/13))=-56/65}}}
ans: a) -56/65