Take a closer look at the part about derivatives and integrals in the textbook (we only learned definite integrals, not indefinite integrals) and master the basic theorems of calculus. Integral is the inverse process of derivative, so it is easy to find any original function. If it is a definite integral, then subtract the function value of the original function of the upper integral limit from the function value of the lower integral limit to find the function value of the function in the interval [a, b].