Math basics › More calculus
Area between curves
Top minus bottom
∫ (upper − lower) dx across the interval. Which one is on top is a question about the graph, not the algebra.
Find where they cross first
The intersection points are usually the limits of integration. Set the two functions equal and solve.
When they swap
If the curves cross inside the interval, split the integral there. One integral with the wrong one on top subtracts area instead of adding it.