Edward C. answered 05/07/20
Caltech Grad for math tutoring: Algebra through Calculus
Sam, you are just taking the integral of y from a1 to a2.
For example, if y = 4x + 1, then an antiderivative of y is
F(x) = 2x2 + x
The integral of y from 1 to 4 is
F(4) - F(1) = 32 + 1 = 33, which is the area under 4x + 1 from 1 to 4.