Evaluating some integrals I need to evaluate two kind of similar integrals
The first one:
$$\lim _{n\to \infty }\int _0^{\frac{\pi }{3}}\:\frac{\sin ^n\left(x\right)}{\sin ^n\left(x\right)+\cos ^n\left(x\right)}dx$$
The second one:
$$\int _0^{2\pi }\:\frac{x\sin ^{100}\left(x\right)}{\sin ^{100}\left(x\right)+\cos ^{100}\left(x\right)}dx$$
For the first one, I got to this form
$$\lim _{n\to \infty }\int _0^{\frac{\pi }{3}}\:\frac{\sin ^n\left(x\right)}{\sin ^n\left(x\right)+\cos ^n\left(x\right)}dx = \frac{\pi }{3}-\int _0^{\frac{\pi }{3}}\:\frac{\cos ^n\left(x\right)dx}{\sin \:^n\left(x\right)+\cos \:^n\left(x\right)}$$
Now, I would make a substitution $u = \frac{\pi}{3}-x$ but it seems it doesn't help in this situation.
I have put the second integral here because I think it should have the same solving concept.

For the first try substituting $x=\frac{2}{3}u$ and divide through by $\sin^n(\frac{2}{3}x)$ top and bottom.
So now we have,
$$\frac{2}{3} \lim_{n \to \infty} \int_{0}^{\frac{\pi}{2}} \frac{1}{1+\cot^n (\frac{2}{3}x)} dx $$
Now for $\frac{2}{3}x \in (0,\frac{\pi}{4})$ we have $\cot \frac{2}{3} x>1$ so there the integrand tends to $0$. This is when $x \in (0,\frac{3\pi}{8})$. Whereas for $x \in (\frac{3\pi}{8},\frac{3\pi}{4})$  we have $\cot \frac{2}{3} x \in (0,1)$ so there the integrand tends to $1$ and thus we have,
$$=\frac{2}{3} \int_{\frac{3}{8}\pi}^{\frac{\pi}{2}} 1 dx$$
$$=\color{red}{\frac{\pi}{12}}$$
For the second integral with $x=u+\pi$ we have,
$$I=\int_{-\pi}^{\pi} (x+\pi)\frac{\sin^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)} dx$$
Notice that $x\frac{\sin^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)}$ is odd. 
$$\color{blue}{I}=\color{blue}{\pi \int_{-\pi}^{\pi} \frac{\sin^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)} dx}$$
$$=\pi \int_{-\pi}^{\pi} \frac{\sin^{100} (x)+\cos^2(x)-\cos^2(x)}{\sin^{100} (x)+\cos^{100} (x)}$$
$$=\pi(2\pi-\int_{-\pi}^{\pi}\frac{\cos^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)} dx)$$
$$=2\pi^2-\color{blue}{\pi\int_{-\pi}^{\pi}\frac{\cos^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)}} dx$$
But,
$$\color{blue}{I}=\pi \int_{-\pi}^{\pi}\frac{\cos^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)} dx$$
$$=\pi \int_{-\pi}^{\pi} \frac{\sin^{100} (x)}{\sin^{100} (x)+\cos^{100} (x)} dx$$
Follows from $x=\frac{\pi}{2}-u$ then $u-\frac{\pi}{2}=v$.
So,
$$I=2\pi^2-I$$
$$I=\color{red}{\pi^2}$$
In fact everything I wrote for this will work for $n$ an even positive integer instead of $100$.
