sums and differnces of different angles

**pogiphilip** · September 21st 2009, 01:47 AM

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

need help

**red_dog** · September 21st 2009, 01:48 AM

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

**pogiphilip** · September 21st 2009, 01:51 AM

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

**red_dog** · September 21st 2009, 02:03 AM

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

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

**pogiphilip** · September 21st 2009, 02:29 AM

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

**mathaddict** · September 21st 2009, 04:34 AM

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 ??

Thread: sums and differnces of different angles