I probably do these a little differently from most people.
I cannot remember the integrals so I so them more from scratch.
\(\displaystyle\int_0^{\frac{1}{6}}\frac{1}{\sqrt{1-9x^2}}dx\)
I am going to use this triangle.
\(cos\theta=\sqrt{1-9x^2}\\ sin\theta=3x\\ x=\frac{sin\theta}{3}\\ \frac{dx}{d\theta}=\frac{cos\theta}{3}\\ dx=\frac{cos\theta}{3}d\theta\)
also
\(when \;x=0 \\ sin\theta=0\\ \theta=0\\~\\ when\;x=1/6\\ sin\theta=3*(1/6)=1/2\\ \theta = \frac{\pi}{6}\)
\(\displaystyle\int_0^{\frac{1}{6}}\frac{1}{\sqrt{1-9x^2}}dx\\ =\displaystyle\int_0^{\frac{1}{6}}\frac{1}{cos\theta}dx\\ =\displaystyle\int_0^{\frac{\pi}{6}}\frac{1}{cos\theta}\frac{cos\theta}{3}d\theta\\ =\displaystyle\int_0^{\frac{\pi}{6}}\frac{1}{3}d\theta\\ =\left[\frac{\theta}{3}\right]_0^{\frac{\pi}{6}}\\ =\frac{\pi}{18} \)
.