Math
posted by Vincent .
Given that cos 13pi/18=sin y, first express 13pi/18 as a sum of pi/2 and an angle, and then apply a trigonometric identitiy to determine the measure of angle y.

13π/18 = 9π/18 + 4π/18 = π/2 + 2π/9
cos 13π/18 = sin 2π/9