int frac{sin x+sin ^3 x}{cos 2 x} ,d x=A cos | vedclass

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(A) Let $I = \int \frac{\sin x+\sin ^3 x}{\cos 2 x} \,d x$. $= \int \frac{\sin x(1+\sin ^2 x)}{\cos 2 x} \,d x$. $= \int \frac{\sin x(1+1-\cos ^2 x)}{

Vedclass · Accepted Answer

$A=\frac{1}{2}, B=\frac{-3}{4 \sqrt{2}}, f(x)=\frac{\sqrt{2} \cos x-1}{\sqrt{2} \cos x+1}$

Vedclass · Answer

(A) Let $I = \int \frac{\sin x+\sin ^3 x}{\cos 2 x} \,d x$. $= \int \frac{\sin x(1+\sin ^2 x)}{\cos 2 x} \,d x$. $= \int \frac{\sin x(1+1-\cos ^2 x)}{2 \cos ^2 x-1} \,d x$. $= \int \frac{\sin x(2-\cos ^2 x)}{2 \cos ^2 x-1} \,d x$. Substitute $\cos x = t$,so $-\sin x \,d x = dt$ or $\sin x \,d x = -dt$. $I = -\int \frac{2-t^2}{2 t^2-1} dt = \int \frac{t^2-2}{2 t^2-1} dt$. $= \frac{1}{2} \int \frac{2 t^2-4}{2 t^2-1} dt = \frac{1}{2} \int \left(1 - \frac{3}{2 t^2-1}\right) dt$. $= \frac{1}{2} \int dt - \frac{3}{2} \int \frac{dt}{(\sqrt{2} t)^2-1^2}$. Using the formula $\int \frac{dx}{x^2-a^2} = \frac{1}{2a} \log |\frac{x-a}{x+a}|$,we get: $= \frac{1}{2} t - \frac{3}{2} \cdot \frac{1}{\sqrt{2}} \cdot \frac{1}{2} \log |\frac{\sqrt{2} t-1}{\sqrt{2} t+1}| + c$. $= \frac{1}{2} \cos x - \frac{3}{4 \sqrt{2}} \log |\frac{\sqrt{2} \cos x-1}{\sqrt{2} \cos x+1}| + c$. Comparing with $A \cos x + B \log |f(x)| + c$,we have $A = \frac{1}{2}$,$B = \frac{-3}{4 \sqrt{2}}$,and $f(x) = \frac{\sqrt{2} \cos x-1}{\sqrt{2} \cos x+1}$.

$\int \frac{\sin x+\sin ^3 x}{\cos 2 x} \,d x=A \cos x+B \log |f(x)|+c$ (where $c$ is a constant of integration). Then the values of $A, B$ and $f(x)$ are:

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