Question

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

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