i have to find the slope of the tangent line at theta = pi/2 of the graph r=2 + 2cos(theta).

i did the x=rcos(Theta) and y=rsin(theta) thing then the dx/d(theta) dy/d(theta) thing, then dy/dx

i ended up with 4cos^2(theta) + 2cos(theta) - 2 for dy and dx = -2sin(theta) - 4cos(theta)sin(theta).

the slope is -2/-2 = 1 but how do you calculate 4cos^2(pi/2)?