if the definite integral $\displaystyle \int_1^3 (x^2+1) dx$ is approximated by using the trapezoid rule with n=4, the error is?
Your textbook or classnotes will have a formula for the size of the maximum error. Please post it and then say what part of it you're having trouble with.
The whole point of the trapezoidal rule is to compute definite integrals numerically and to know what the maximum error is in the result for a given value of n.