Write down the integral
∫xexdx The integrand is a product of two different kinds of function, so integration by parts is appropriate.
Recall the integration by parts formula
∫udxdvdx=uv−∫vdxdudx This rewrites the integral of a product in terms of a simpler integral.
Choose u using LIATE
LIATE (logs, inverse-trig, algebraic, trig, exponential) selects the factor to differentiate; it should become simpler.
Choose dxdv (the other factor)
dxdv=ex The remaining factor is the part we integrate.
Differentiate u to find dxdu
dxdu=1 Differentiate the chosen u.
Integrate dxdv to find v
Integrate the other factor; no constant of integration is needed at this stage.
Collect the four ingredients
u=x,dxdu=1,dxdv=ex,v=ex Having every piece ready makes the substitution reliable.
Substitute into the formula
∫xexdx=xex−∫exdx Replace uv and ∫vdxdudx using the pieces above.
Look at the remaining integral
∫exdx This integral is simpler than the original.
Evaluate the remaining integral
∫exdx=ex Integrate again — a second application of integration by parts may be required.
Combine the two parts
xex−(ex) Subtract the value of the remaining integral from uv.
Simplify
(x−1)ex Collect the terms into a single expression.
Add the constant of integration
(x−1)ex+c An indefinite integral must include the arbitrary constant +c.
Check by differentiating
dxd((x−1)ex)=xex Differentiating the answer returns the original integrand, confirming the result.
Select the correct formula
∫udv=uv−∫vdu This is the standard statement of integration by parts.