When to Use Variable Substitution with Integrals
Variable substitution comes in handy for some integrals. The anti-differentiation formulas plus the Sum Rule, Constant Multiple Rule, and Power Rule allow you to integrate a variety of common functions. But as functions begin to get a little bit more complex, these methods become insufficient. For example, these methods don’t work on the following:
To evaluate this integral, you need some stronger medicine. The sticking point here is the presence of the constant 2 inside the sine function. You have an anti-differentiation rule for integrating the sine of a variable, but how do you integrate the sine of a variable times a constant?
The answer is variable substitution, a five-step process that allows you to integrate where no integral has gone before. Here are the steps:
Declare a variable u and set it equal to an algebraic expression that appears in the integral, and then substitute u for this expression in the integral.
Differentiate u to find du/dx.
This gives you the differential du = '(x)dx.
Make another substitution to change dx and all other occurrences of x in the integral to an expression that includes du.
Integrate using u as your new variable of integration.
Express this answer in terms of x.