利用定义计算定积分 上2下-1 x^2dx

divide (-1,2) into n equal interval (3/n) -1+3/n , -1+6/n,...., 2 ∫(-1->2) x^2dx =lim(n->无穷) summation(i:1->n) f（ξi）△ xi =lim(n->无穷) summation(i:1->n) (3/n) (-1+ 3i/n)^2 = lim(n->无穷) summation(i:1->n) (3/n) ( 1 - 6i/n + 9i^2/n^2 ) = lim(n->无穷) 3 (1+ 3(n+1)/n + (3/2) (n+1)(2n+1)/n^2 ) =3