As described in the previous example, when the integrand includes certain radical expressions, a set of trigonmetric substitutions can be used. We follow the 'recipe' of the substitution rule. This 'recipe' helps us to convert the function from one expressed in terms of 'x' to a function of angle Θ. Once the conversion has been made, basic trigonometric identities are used to solve for the variable values. We then, solve the integration for the function expressed in terms of Θ, and finally replace the original variables into the result. For example, here we evaluate:
$\int \frac{dx}{\sqrt{36+{x}^{2}}}$