# Thread: Need help!....again. Evaluating a trig function

1. ## Need help!....again. Evaluating a trig function

If sinx=-12/13 and x is in Quadrant IV, and cosy=-7/25, y in Quadrant II, find the value of sin(x-y) and cos(x+y).

Thank you!

2. Draw the triangles out in both quadrants. Use pythagoras' theorem to solve for the hypotenuse of both triangles. Now you have all the necessary information to get sinx, cosx, sin y, and cos y. With these values, simply plug them into the formulae for sin(x-y) and cos(x+y):

$\displaystyle \sin (x-y) = \sin x \cos y - \cos x \sin y$
$\displaystyle \cos (x + y) = \cos x \cos y - \sin x \sin y$

3. Thank you much! Does cos(x+y)= -288/325 and sin(x-y)=204/325 sound right?

4. Sorry, I didn't mean hypotenuse earlier.

Hmm. That doesn't seem right. What did you get for sinx, cosx, siny, and siny?

5. sinx= -12/13
siny= -24/25
cosx= 5/13
cosy= -7/25

6. siny shouldn't be negative. It's in quadrant II so siny must be positive while cosy must be negative. Hope that works out for ya!