Since cos theta is 15/17, let a right triangle have a hypotenuse
of 17 and a leg of 15. This means that the other is 8.
Sin 2 theta is a double angle identity.
Sin 2 theta = 2 sin
theta cos theta = 2(8/17)(15/17) = 240/289
Cos 2theta = cos^2 – sin^2 = (15/17)^2 – (8/17)^2 = 161/289
