# Thread: integration of product of two dissimlar expressions

1. ## integration of product of two dissimlar expressions

I have to integrate by substitution:

$\displaystyle \int csc^2(3x+5)\frac{\pi}{4}dx$ between 8/3 and 2

tried making (3x+5)=u to give:

$\displaystyle \int csc^2\frac{\pi u}{4}\frac{du}{3}$

I'm not sure if this is correct though, and I don't know how to integrate trig functions when the variable has a coefficient. Please help.

2. So the $\displaystyle \frac{\pi}{4}$ is inside the csc? That is, it is $\displaystyle csc^2(\frac{\pi}{4}(3x+5))$ which is not quite what you wrote.

Once you have $\displaystyle \frac{1}{3}\int csc^2\frac{\pi u}{4}du$ make the further substitution $\displaystyle v= \frac{\pi u}{4}$ or, equivalently, make the original substitution $\displaystyle u= \frac{\pi}{4}(3x+ 5)$ rather than just u= 3x+ 5.

3. Hi, the pi/4 is within the csc, yes. The answer is supposed to be 0 but i just cant seem to get that.

4. With the substitution $\displaystyle u= \frac{\pi}{4}(3x+ 5)$, $\displaystyle du= \frac{3\pi}{4}dx$ so $\displaystyle dx= \frac{4}{3\pi}du$. When x= 8/3, $\displaystyle u= \frac{\pi}{4}(8+ 5)= \frac{13\pi}{4}$ and when x= 2, $\displaystyle u= \frac{\pi}{4}(6+ 5)= \frac{11\pi}{4}$. The integral becomes
$\displaystyle \frac{4}{3\pi}\int_{11\pi/4}^{13\pi/4} csc^2(u)du$

Now, the anti-derivative of $\displaystyle csc^2(u)$ is $\displaystyle - cot(u)$. Evaluated at $\displaystyle \frac{13\pi}{4}$ that is 1 and at $\displaystyle \frac{11\pi}{4}$ it is -1. The value of the integral is NOT 0, it is $\displaystyle \frac{8}{3\pi}$.