Question
If $$u = f\left( {{x^3}} \right),\,v = g\left( {{x^2}} \right),\,f'\left( x \right) = \cos \,x$$ and $$g'\left( x \right) = \sin \,x$$ then $$\frac{{du}}{{dv}}$$ is :
A.
$$\frac{3}{2}x\,\cos \,{x^3}.\,{\text{cosec}}\,{x^2}$$
B.
$$\frac{2}{3}\sin \,{x^3}.\sec \,{x^2}$$
C.
$$\tan \,x$$
D.
none of these
Answer :
$$\frac{3}{2}x\,\cos \,{x^3}.\,{\text{cosec}}\,{x^2}$$
Solution :
$$\frac{{du}}{{dv}} = \frac{{\frac{{du}}{{dx}}}}{{\frac{{dv}}{{dx}}}} = \frac{{f'\left( {{x^3}} \right).3{x^2}}}{{g'\left( {{x^2}} \right).2x}} = \frac{{\cos \,{x^3}.3{x^2}}}{{\sin \,{x^2}.2x}} = \frac{3}{2}x\,\cos \,{x^3}.{\text{cosec}}\,{x^2}$$