Question
If $$\int {f\left( x \right)dx} = g\left( x \right) + c,$$ then $$\int {{f^{ - 1}}\left( x \right)dx} $$ is equal to :
A.
$$x{f^{ - 1}}\left( x \right) + C$$
B.
$$f\left( {{g^{ - 1}}\left( x \right)} \right) + C$$
C.
$$x{f^{ - 1}}\left( x \right) - g\left( {{f^{ - 1}}\left( x \right)} \right) + C$$
D.
$${g^{ - 1}}\left( x \right) + C$$
Answer :
$$x{f^{ - 1}}\left( x \right) - g\left( {{f^{ - 1}}\left( x \right)} \right) + C$$
Solution :
Let $$I = \int {{f^{ - 1}}\left( x \right)dx} $$ and $${f^{ - 1}}\left( x \right) = t \Rightarrow x = f\left( t \right) \Rightarrow dx = f'\left( t \right)\,dt$$
Put value of $$dx$$ and $${f^{ - 1}}\left( x \right)$$ in $$I,$$ we get $$I = \int {tf'\left( t \right)dt} $$
Now, integrate it by parts, $$I = tf\left( t \right) - \int {f\left( t \right)} dt$$
Given, $$\int {f\left( x \right)dx = g} \left( x \right) + C$$
$$\therefore \,I = tf\left( t \right) - \left[ {g\left( t \right)} \right] + C$$
Now, by putting value of $$t,\,f\left( t \right)$$ and $$g\left( t \right)$$ we get,
$$I = x{f^{ - 1}}\left( x \right)g\left[ {{f^{ - 1}}\left( x \right)} \right] + C$$