Question
Let $${I_n} = \int {{{\tan }^n}x\,dx,\,\left( {n > 1} \right).} $$ $${I_4} + {I_6} = a\,{\tan ^5}x + b{x^5} + C,$$ where $$C$$ is constant of integration, then the ordered pair $$\left( {a,\,b} \right)$$ is equal to :
A.
$$\left( { - \frac{1}{5},\,\,0} \right)$$
B.
$$\left( { - \frac{1}{5},\,\,1} \right)$$
C.
$$\left( { \frac{1}{5},\,\,0} \right)$$
D.
$$\left( {\frac{1}{5},\,\, - 1} \right)$$
Answer :
$$\left( { \frac{1}{5},\,\,0} \right)$$
Solution :
$$\eqalign{
& {I_n} = \int {{{\tan }^n}x\,dx,\,n > 1} \cr
& {\text{Let }}I = {I_4} + {I_6} \cr
& = \int {\left( {{{\tan }^4}x + {{\tan }^6}x} \right)dx} \cr
& = \int {{{\tan }^4}x\,\,{{\sec }^2}x\,\,dx} \cr
& {\text{Let}}\,\,\,\tan \,x = t \cr
& \Rightarrow {\sec ^2}x\,dx = dt \cr
& \therefore I = \int {{t^{^4}}dt} = \frac{{{t^5}}}{5} + C \cr
& = \frac{1}{5}{\tan ^5}x + C \cr} $$
$$ \Rightarrow $$ On comparing, we have
$$a = \frac{1}{5},\,\,\,\,b = 0$$