3)求和近似和 A=∑A41∑f()Ax 4)取极限.令=max{Ax},则曲边梯形面积 1≤i<n A=1m244 ->0i =1im∑f()Ax 2→>0 o a x1 xixi 6x 53) 求和(近似和).  = =  n i A Ai 1  =   n i i i f x 1 ( ) 4) 取极限. 令 max{ }, 1 i i n = x    则曲边梯形面积  → = =  n i A Ai 1 0 lim   → = =  n i i i f x 1 0 lim ( )  a b x y o 1 x i x i−1 x  i