Question
The integral $$\int {{{\sec }^{\frac{2}{3}}}x\,{\text{cose}}{{\text{c}}^{\frac{4}{3}}}x\,dx} $$ is equal to:
(Here $$C$$ is a constant of integration)
A.
$$ - 3\,{\tan ^{ - \,\frac{1}{3}}}x + C$$
B.
$$ - \frac{3}{4}\,{\tan ^{ - \,\frac{4}{3}}}x + C$$
C.
$$ - 3\,{\cot ^{ - \,\frac{1}{3}}}x + C$$
D.
$$3\,{\tan ^{ - \,\frac{1}{3}}}x + C$$
Answer :
$$ - 3\,{\tan ^{ - \,\frac{1}{3}}}x + C$$
Solution :
$$\eqalign{
& I = \int {\frac{{dx}}{{{{\left( {\sin \,x} \right)}^{\frac{4}{3}}}.{{\left( {\cos \,x} \right)}^{\frac{2}{3}}}}}} \cr
& \Rightarrow I = \int {\frac{{dx}}{{{{\left( {\frac{{\sin \,x}}{{\cos \,x}}} \right)}^{\frac{4}{3}}}.{\cos^2}\,x}}} \cr
& \Rightarrow I = \int {\frac{{{{\sec }^2}x}}{{{{\left( {\tan \,x} \right)}^{\frac{4}{3}}}}}dx} \cr
& {\text{Put }}\tan \,x = t \Rightarrow {\sec ^2}x\,dx = dt \cr
& \therefore \,I = \int {\frac{{dt}}{{{t^{\frac{4}{3}}}}}} \Rightarrow I = \frac{{ - 3}}{{{t^{\frac{1}{3}}}}} + C \cr
& \Rightarrow I = \frac{{ - 3}}{{{{\left( {\tan \,x} \right)}^{\frac{1}{3}}}}} + C \cr
& \Rightarrow I = - 3{\mkern 1mu} {\tan ^{ - {\kern 1pt} \frac{1}{3}}}x + C \cr} $$