I am attempting to evaluate the integral $$\int_0^{\frac{1}{6}}\frac{x^5}{(36x^2+1)^{\frac{3}{2}}}\text{d}x\,\,\,.$$ Part of the solution is shown in the below image:
I understand how to substitute the $x$-value, but down at the bottom of the image, when it converts the boundaries $a=0$ and $b=\frac{1}{6}$ to radians, where the hell does the $1$ come from for the upper bound?
The way I understand the conversion is that to get radians or $\theta$, I just get the inverse of whatever trigonometric substitution I used for $x$ with the boundaries as inputs.