# Math Help - how do I do this trig addition formula problem?

1. ## how do I do this trig addition formula problem?

Angle A is acute.
I have a list of trig formulas

here is the problem...
19. If sin A = 5/13, find sin 2A and cos 2A

2. Originally Posted by RaphaelB30
Angle A is acute.
I have a list of trig formulas

here is the problem...
19. If sin A = 5/13, find sin 2A and cos 2A
$\cos(2A) = 1 - 2\sin^2{A}$

$\sin(2A) = 2\sin{A}\cos{A}$ ... use a reference triangle or the Pythagorean identity to find $\cos{A}$

3. Originally Posted by RaphaelB30
Angle A is acute.
I have a list of trig formulas

here is the problem...
19. If sin A = 5/13, find sin 2A and cos 2A
sin(x) = opp/hyp so you construct a right angled triangle with the opposite 5 and hypotenuse 13. You can then find the adjacent side easily enough and thus find cos(x).

For sin(2A) and cos(2A) use an identity