Use cos(A+B)=cosAcosB-sinAsinB and let A=B so cos(A+A)=cosAcosA-sinAsinA this means cos(2A)=cos2A-sin2A and since cos2A+sin2A=1, cos2A=1-sin2
x3 - x - y3 - 20 = 0 Find dy/dx. Differentiate with respect to x.
3x2 - 1 - 3y2(dy/dx) = 0Therefore: dy/dx = (3x