- #1
Dewgale
- 98
- 9
Homework Statement
[/B]
Evaluate the integral $$\int_{y=0}^{y=1} \int_{x=y}^{x=1} sin(x^2) \, dx \, dy$$
Homework Equations
N/A
The Attempt at a Solution
We know right away that ##sin(x^2)## has no elementary anti-derivative. Therefore, I analyzed the Maclaurin series of ##sin(x)##.
$$sin(x)\ =\ x\ -\ \frac{x^3}{3!}\ +\ \frac{x^5}{5!}\ -\ \frac{x^7}{7!}\ +\ \dots$$
$$sin(x)\ =\ \sum_{n=0}^\infty \frac{(-1)^n(x)^{2n+1}}{(2n+1)!}$$
Therefore,
$$sin(x^2)\ =\ \sum_{n=0}^\infty \frac{(-1)^n(x)^{4n+2}}{(2n+1)!}$$
I then substituted this sum into the original integral.
$$\int_{y=0}^{y=1} \int_{x=y}^{x=1} \sum_{n=0}^\infty \frac{(-1)^n(x)^{4n+2}}{(2n+1)!} \, dx \, dy$$
As ##\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!}## is a constant, we can bring it outside of the integral. Then the integral becomes
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \int_{y=0}^{y=1} \int_{x=y}^{x=1} x^{4n+2} \, dx \, dy$$
I then integrated with respect to x, and got
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \int_{y=0}^{y=1} \big( \left. \frac{x^{4n+3}}{4n+3} \right|_y^1 \big) \, dy$$
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \int_{y=0}^{y=1} \big( \frac{1^{4n+3}}{4n+3}\ -\ \frac{y^{4n+3}}{4n+3} \big) \, dy$$
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \frac{1}{4n+3} \int_{y=0}^{y=1} \big(1^{4n+3}\ -\ y^{4n+3} \big) \, dy$$
I then integrated with respect to y.
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \frac{1}{4n+3} \big( (1^{4n+3})\ -\ \frac{1^{4n+4}}{4n+4} \big)$$
$$\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \frac{1^{4n+3}}{4n+3} \ -\ \sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!} \frac{1}{4n+3} \frac{1^{4n+4}}{4n+4}$$
Now here is where I hit a snag. I can brute force calculate using Wolfram|Alpha to show that the first series converges to ##\approx \ 0.310268,## and the second series converges to ##\approx \ 0.08042##. Therefore, the integral converges to ##0.310268\ -\ 0.08042\ \approx\ 0.2299848##
This seems remarkably inelegant though, and I'm sure the prof isn't ok with the brute forcing. I'm just not sure how to progress beyond this point. Any help is appreciated greatly!