Explanation
Let $$I= \int \cos (\ln x) dx$$ $$\displaystyle I= x \cos (\ln x) + \int x \sin (\ln x) \frac{1}{x}dx$$ $$\Rightarrow I= x \cos (\ln x) + \int \sin (\ln x) dx$$ $$\displaystyle=x \cos (\ln x) + [x \sin (\ln x)-\int x \cos (\ln x).\frac{1}{x}dx]$$ $$\Rightarrow 2I=x(\cos (\ln x)+\sin (\ln x))$$ $$\therefore \displaystyle I=\frac{x}{2} (\cos (\ln x)+\sin (\ln x))+c$$
None of the above
$$\displaystyle\int {\frac{dx}{ { x }^{ n }\left( \sqrt { ax+b } \right) } } = -\frac { \sqrt { ax+b } }{ \left( n-1 \right) b{ x }^{ n-1 } } -A\int { \frac { dx }{ { x }^{ n-1 }\left( \sqrt { ax+b } \right) } } + C$$
$$\Rightarrow\displaystyle \int { \frac { 1+Ax }{ { x }^{ n }\left( \sqrt { ax+b } \right) } } dx = -\frac { \sqrt { ax+b } }{ \left( n-1 \right) b{ x }^{ n-1 } } + C$$
Differentiating on both sides,
$$\dfrac{1+Ax}{x^n(\sqrt{ax+b})} =- \dfrac{x^{n-1}\dfrac{a}{2}(ax+b)^{-\frac12}-(ax+b)^{\frac12}(n-1)x^{n-2}}{b(n-1)x^{2n-2}}$$
$$\Rightarrow\dfrac{1+Ax}{x^n\sqrt{ax+b}}=-\dfrac{ax-2(n-1)(ax+b)}{b(n-1)2x^n\sqrt{ax+b}}$$
Simplifying further, it gives
$$ A = \dfrac { 2n-3 }{ 2n-2 } \dfrac { a }{ b } $$
Please disable the adBlock and continue. Thank you.