The equation of the unit circle is x^2 + y^2 = 1 If x = 5/8, then solving for y (5/8)^2 + y^2 = 1 y^2 = 1 - 25/64 y^2 = 39/64 y = sqrt(39/64) We use the positive square root because positive x-coordinate and positive y-coordinate is in the first quadrant. Therefore the answer to this question is sqrt(39)/8