Posted by **anna** on Sunday, January 3, 2010 at 2:18pm.

how do i find cosX if i am given sin x= -4/5 and tan <o???

- trig -
**Reiny**, Sunday, January 3, 2010 at 2:55pm
Isn't the sine negative in the III and IV quadrants, and isn't the tangent negative in the II and IV quadrant.

So clearly our angle x must be in IV

Make a triangle using sinx = 4/5

The third side (the adjacent) would be 3 and cosx = 3/5, but we are in quadrant IV, and in IV the cosine is positive, so ..

cosx = 3/5

- trig -
**bobpursley**, Sunday, January 3, 2010 at 2:56pm
cos^2x+ sin^2 x=1

the tangent being negative tells you which of the two answers is correct.

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