>>776
sin(x) = a sin(y)
cos(x) = b cos(y)

sin(x)^2 +cos(x)^2 = (a^2) sin(y)^2 + (b^2) cos(y)^2 =1

(a^2) tan(y)^2 +(b^2) = 1/cos(y)^2 = 1+tan(y)^2

tan(y)^2 = (1-(b^2))/(a^2)