+23246 If cos(a) = 12/13, then the adjacent side is 12 and the hypotenuse is 13 (or a multiple of these).
If the adjacent side is 12 and the hypotenuse is 13, the opposite side is 5 (Pythagorean Theorem).
However, since the angle is in the 4th quadrant, we'll have to make the opposite side = -5.
sin(2a) = 2·sin(a)·cos(a) = 2·(-5/13)·(12/13) = .......
cot(2a) = 1 / tan(2a) = 1 / (-5/12) = ........
