Question
The integral $$\int {\frac{{{{\sec }^2}x}}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{9}{2}}}}}dx} ,$$ equals (for some arbitrary constant $$K$$ )
A.
$$ - \frac{1}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} - \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K$$
B.
$$\frac{1}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} - \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K$$
C.
$$ - \frac{1}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} + \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K$$
D.
$$\frac{1}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} + \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K$$
Answer :
$$ - \frac{1}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} + \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K$$
Solution :
$$\eqalign{
& I = \int {\frac{{{{\sec }^2}x}}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{9}{2}}}}}dx} \cr
& {\text{Let }}\sec \,x + \tan \,x = t \cr
& \Rightarrow \sec \,x - \tan \,x = \frac{1}{t} \cr
& \Rightarrow \sec \,x = \frac{1}{2}\left( {t + \frac{1}{t}} \right) \cr
& {\text{Also }}\sec x\left( {\sec \,x + \tan \,x} \right)dx = dt \cr
& \Rightarrow \sec \,x\,dx = \frac{{dt}}{t} \cr
& \therefore I = \frac{1}{2}\int {\frac{{\left( {t + \frac{1}{t}} \right)dt}}{{{t^{\frac{9}{2}}}.t}}} \cr
& = \frac{1}{2}\int {\left( {{t^{ - \,\frac{9}{2}}} + {t^{ - \,\frac{{13}}{2}}}} \right)} dt \cr
& = \frac{1}{2}\left[ {\frac{{{t^{ - \,\frac{9}{2}\, + \,1}}\,}}{{ - \frac{9}{2} + 1}} + \frac{{{t^{ - \,\frac{{13}}{2}\, + \,1}}}}{{ - \frac{{13}}{2} + 1}}} \right] + K \cr
& = \frac{{ - 1}}{7}{t^{ - \,\frac{7}{2}}} - \frac{1}{{11}}{t^{ - \,\frac{{11}}{2}}} + K \cr
& = - \frac{1}{{7{t^{\,\frac{7}{2}}}}} - \frac{1}{{11{t^{\,\frac{{11}}{2}}}}} + K \cr
& = - \frac{1}{{{t^{\frac{{11}}{2}}}}}\left( {\frac{1}{{11}} + \frac{{{t^2}}}{7}} \right) + K \cr
& = \frac{{ - 1}}{{{{\left( {\sec \,x + \tan \,x} \right)}^{\frac{{11}}{2}}}}}\left\{ {\frac{1}{{11}} + \frac{1}{7}{{\left( {\sec \,x + \tan \,x} \right)}^2}} \right\} + K \cr} $$