Asked by Math

tan A=-4/5;A in Quadrant 2 and cos B=-5/13: B in Quadrant 3.

What is the exact value of the expression cos(A+B)?
Answered by Steve
draw your triangles in standard position. It will be clear that

sinA = 4/√41
cosA = -5/√41

sinB = -12/13
cosB = -5/13

Now just plug that into the formula for cos(A+B)

Although I suspect a typo...
Answered by Manuel
73*41^1/2/533
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