- Simpson's rule is used to estimate definite integrals by dividing the area under the curve into an even number of strips of equal width. - The formula fits quadratic curves to points along the strips to estimate the area. - The formula is (n/3)[y1 + 4y2 + 2y3 + 4y4 + ... + 4yn-1 + yn] where n is the even number of strips and yi are the function values along the strips. - Increasing the number of strips n improves the accuracy of the approximation.