find derivative using chain rule
x = cos(z) z = 12 teta dy/dtheta = (dy/dz) (dz/dtheta) = (dy/dx) (dx/dz) (dz/dtheta) = 3 x^2 [-sin(z)] 12 = -36 cos^2(z) sin(z) = -36 cos^2(12 theta) sin(12 theta)
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