# Thread: sums and differnces of different angles

1. ## sums and differnces of different angles

the question is:
find the exact value of
sin(x-y) when sinx=8/17 and cos=5/13

need help

2. Use the formula $\sin(x-y)=\sin x\cos y-\sin y\cos x$.

3. Originally Posted by red_dog
Use the formula $\sin(x-y)=\sin x\cos y-\sin y\cos x$.
but you only have sinx and cosy you dont have siny and cosx

4. $\cos x=\pm\sqrt{1-\sin^2x}, \ \sin y=\pm\sqrt{1-\cos^2y}$

To choose the sign before the radical, you have to know in which quadrant are x and y.

5. i wasnt taught the radical yet, is there another way like guess and check?

6. Originally Posted by pogiphilip
i wasnt taught the radical yet, is there another way like guess and check?
Hi

Do you know the domain for x and y ? ie 0<x<90 ??