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.

## Answer This Question

## Related Questions

- Trig Identities - Proving identities: 1) 1+ 1/tan^2x = 1/sin^2x 2) 2sin^2 x-1 = ...
- Precalculus/Trig - I can't seem to prove these trig identities and would really ...
- Trig....... - I need to prove that the following is true. Thanks (2tanx /1-tan^x...
- Trig - Given: cos u = 3/5; 0 < u < pi/2 cos v = 5/13; 3pi/2 < v < ...
- Trigonometry - Prove the following trigonometric identities. please give a ...
- Math-Trig - 1. Create an algebraic expression for sin(arccosx-arcsin3x) 2. The ...
- trig - find sin 2x if tan x=2 cosx>0
- Trig Identities - Prove the following identities: 13. tan(x) + sec(x) = (cos(x...
- TRIG.............. - Q.1 Prove the following identities:- (i) tan^3x/1+tan^2x + ...
- trig 26 - simplify to a constant or trig func. 1. sec Ču-tan Ču/cos Čv+sin Čv ...

More Related Questions