Describe the type of integral for which integration by parts is useful.

When two functions are multiplied

Step-by-step explanation:

Integration by parts is useful when there is a product of two functions

for example; ∫2xcosxdx

The two functions are 2x and cosx which can not be multiplied if we don't know the values of x.

the functions are represented by u and dv and the integration is calculated as follows.

∫udv = uv - ∫vdu