1. ## Triangles with trigonometry

If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

I do not know how to go about this question. All I have done is found that the reference angle is about 22.6 degrees, but I have to find the exact value, how do I solve this?

2. Originally Posted by bhuang
If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

I do not know how to go about this question. All I have done is found that the reference angle is about 22.6 degrees, but I have to find the exact value, how do I solve this?
since $A$ is obtuse, $\cos{A} = -\frac{12}{13}$

$\sin(2A) = 2\sin{A}\cos{A}$

3. how did you get the ratio for cosine?

4. Originally Posted by bhuang
how did you get the ratio for cosine?
I sketched a reference triangle in quad II ...

since $\sin{A} = \frac{5}{13}$ , opposite side = 5 , hypotenuse = 13

using Pythagoras ... adjacent side = -12

$\cos{A} = -\frac{12}{13}$