# 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*= ƒ'(*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.*