Question

Find the exact value of the trigonometric expression given that sin u = − 8 17 and cos v = − 24 25 . (Both u and v are in Quadrant III.) csc(u − v)

Answer 1

Answer: sin u = -5/13 and cos v = -15/17

Step-by-step explanation:

The nice thing about trig, a little information goes a long way. That’s because there is a lot of geometry and structure in the subject. If I have sin u = opp/hyp, then I know opp is the opposite side from u, and the hypotenuse is hyp, and the adjacent side must fit the Pythagorean equation opp^2 + adj^2 = hyp^2.

So for u: (-5)^2 + adj^2 = 13^2, so with what you gave us (Quad 3),

==> adj of u = -12 therefore cos u = -12/13

Same argument for v: adj = -15,

opp^2 + (-15)^2 = 17^2 ==> opp = -8 therefore sin v = -8/17

The cosine rule for cos (u + v) = (cos u)(cos v) - (sin u)(sin v) and now we substitute: cos (u + v) = (-12/13)(-15/17) - (-5/13)(-8/17)

I am too lazy to do the remaining arithmetic, but I think we have created a way to approach all of the similar problems.