# Thread: Pretty simple trigonometry but need help

1. ## Pretty simple trigonometry but need help

suppose a right angle triangle with angle theta. Given cos(90-theta) = 5/[3(11)^0.5], find the exact value of cos(theta).

2. $\cos (a - b) = \cos a \cos b + \sin a \sin b$

$\cos (90 - \theta) = \cos 90 \cos \theta + \sin 90 \sin \theta =$

$\sin \theta$

Use this and the fact that

$\sin^2 \theta + \cos^2 \theta = 1$

3. thanks iceman i finally see the light haha

4. I don't think you need to get that complicated. Just use the definitions of sine and cosine within the right triangle with sides x, y, r, and the fact that if one angle is A, the other is 90 - A, and vice versa. So cos(90 - A) = x/r = sin(A) directly.