We have y=(5x)3cos2x
Taking logarithm on both the sides, we obtain
logy=3cos2xlog5x
Differentiating both sides with respect to x, we obtain
1ydydx=3[log5xddx(cos2x)+cos2xddx(log5x)]
⇒ddx=3y[log5x(−sin2x).ddx(2x)+cos2x.15xddx(5x)]
⇒dydx=3x[−2sin2xlog5x+cos2xx]
⇒dydx=3y[3cos2xx−6sin2xlog5x]
∴dydx=(5x)3cos2x[3cos2xx−6sin2xlog5x]