Question
If $$\int {{{\log }_e}\left( {\sqrt {1 - x} + \sqrt {1 + x} } \right)} dx = x\,{\log _e}\left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) + g\left( x \right) + C.$$
Then $$g\left( x \right) = ?$$
A.
$$x - {\sin ^{ - 1}}x$$
B.
$${\sin ^{ - 1}}x - x$$
C.
$$x + {\sin ^{ - 1}}x$$
D.
$${\sin ^{ - 1}}x - {x^2}$$
Answer :
$${\sin ^{ - 1}}x - x$$
Solution :
$$I = \int {{{\log }_e}\left( {\sqrt {1 - x} + \sqrt {1 + x} } \right)} .1\,dx$$
Integrating by parts taking 1 as the second function.
$$\eqalign{
& I = \log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right)x - \int {\frac{1}{{\sqrt {1 - x} + \sqrt {1 + x} }}\left[ { - \frac{1}{{2\sqrt {1 - x} }} + \frac{1}{{2\sqrt {1 + x} }}} \right]\left( x \right)dx} \cr
& \,\,\,\,\, = x\,\log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) - \frac{1}{2}\int {\frac{{\sqrt {1 - x} - \sqrt {1 + x} }}{{\sqrt {1 - x} + \sqrt {1 + x} }}.\frac{1}{{\sqrt {1 - {x^2}} }}.x\,dx} \cr
& \,\,\,\,\, = x\,\log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) - \frac{1}{2}\int {\frac{{\left( {1 - x} \right) + \left( {1 + x} \right) - 2\sqrt {1 - {x^2}} }}{{\left( {1 - x} \right) - \left( {1 + x} \right)}}} .\frac{1}{{\sqrt {1 - {x^2}} }}.x\,dx \cr
& \,\,\,\,\, = x\,\log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) - \frac{1}{2}\int {\frac{{\sqrt {1 - {x^2}} - 1}}{{\sqrt {1 - {x^2}} }}} dx \cr
& \,\,\,\,\, = x\,\log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) - \frac{1}{2}\left[ {\int {1\,dx - \int {\frac{1}{{\sqrt {1 - {x^2}} }}} } dx} \right] \cr
& \,\,\,\,\, = x\,\log \left( {\sqrt {1 - x} + \sqrt {1 + x} } \right) + \frac{1}{2}\left[ {{{\sin }^{ - 1}}x - x} \right] + C \cr
& \therefore \,f\left( x \right) = x,\,g\left( x \right) = {\sin ^{ - 1}}x - x \cr} $$