# Thread: Help with solving these Trig Identities... Part 3

1. ## Help with solving these Trig Identities... Part 3

How do I solve:

Using Double Angle identities

1. (2 tan 15) / (1 - tan^2 15)
2. sin(pi/6)cos(pi/6)
3. cos^2 (2x) - sin^2 (2x)

1. [sq.root(3)] / 3
2. [sq.root(3)] / 4
3. cos 4x

Thank you very much for your time! =)

2. Use this:

1) $\frac{2\tan x}{1-\tan^2x}=\tan 2x$

2) $\sin\frac{\pi}{6}=\frac{1}{2}, \ \cos\frac{\pi}{6}=\frac{\sqrt{3}}{2}$

3) $\cos^2a-\sin^2a=\cos 2a$