# Proof integrals

## Homework Statement

a) Solve:$$^{Pi}_{0}$$$$\int$$$$\frac{sin(x)}{1 + cos²x}$$dx
b) Proof that for each f, continuous in [0, a], $$^{a}_{0}$$$$\int$$$${f(x)}$$dx = $$^{a}_{0}$$$$\int$$$${f(a-x)}$$dx
c) Use a and b to solve $$^{Pi}_{0}$$$$\int$$$$\frac{x sin(x)}{1 + cos²x}$$dx

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## The Attempt at a Solution

a) t = cos(x)
dt/dx = sin(x)
dt = sin(x)*dx
$$^{Pi}_{0}$$$$\int$$$$\frac{sin(x)}{1 + cos²x}$$dx
= $$^{1}_{-1}$$$$\int$$$$\frac{dt}{1 + t²}$$dt
= arctan(1)-arctan(-1) = Pi/2

b) t = a - x
dt/dx = -1
-dt = dx
$$^{0}_{a}$$$$\int$$$${-f(t)}$$dt
= $$^{a}_{0}$$$$\int$$$${f(t)}$$dt
= $$^{a}_{0}$$$$\int$$$${f(x)}$$dx

c) I have no idea to start this should I replace x with Pi-x, I tried this but i'm not getting any further

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Mark44
Mentor
Tips:
1. Use only one pair of [ tex] and [ /tex] tags per line. Your stuff will format much more nicely that way.
2. For a definite integral, do it like this:
[ tex]\int _{0} ^ {\pi} f(x) dx [ /tex]
3. Don't use small font superscripts inside LaTeX code. They don't get rendered. For example, I didn't realize that you had 1 + cos2x in the denominator until I looked at your LaTeX.
4. For Greek letters, precede them with a backslash; e.g., \pi, \alpha, etc. Note that the names are case-sensitive. If you capitalize the name (like \Pi), you get the upper-case form of the letter.

## Homework Statement

a) Solve:$$\int ^{\pi}_{0} \frac{sin(x)dx}{1 + cos^2(x)}$$
b) [STRIKE]Proof[/STRIKE] Prove that for each f, continuous in [0, a], $$\int ^{a}_{0}f(x)dx = \int ^{a}_{0}f(a-x) dx$$
c) Use a and b to solve $$\int ^{\pi}_{0}\frac{x sin(x)}{1 + cos^2(x)}dx$$

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## The Attempt at a Solution

a) t = cos(x)
dt/dx = sin(x)
dt = sin(x)*dx
$$\int^{\pi}_{0}\frac{sin(x)}{1 + cos^2x}dx$$
= $$\int^{1}_{-1}\frac{dt}{1 + t^2}dt$$
= arctan(1)-arctan(-1) = Pi/2

b) t = a - x
dt/dx = -1
-dt = dx
$$\int^{0}_{a}-f(t)dt$$
= $$\int ^{a}_{0}f(t)dt$$
= $$\int^{a}_{0}f(x) dx$$

c) I have no idea to start this should I replace x with Pi-x, I tried this but i'm not getting any further
I need to run now, but will take a closer look at this later this afternoon.

Last edited:
Dick
Homework Helper
Yes, replace x by pi-x and equate the two integrals. Then solve for the integral you are after. Thanks for unveiling the cos(x)^2, Mark44, without the square both integrals diverge.

Yes, replace x by pi-x and equate the two integrals. Then solve for the integral you are after. Thanks for unveiling the cos(x)^2, Mark44, without the square both integrals diverge.
Sorry for my bad Latex use

Sorry, but I don't really understand which integrals I have to equate.

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