表达式3: "S" Subscript, "u" "m" , Baseline equals Start sum from "i" equals 1 to "n" , end sum, "f" left parenthesis, "a" plus StartFraction, left parenthesis, 2 "i" minus 1 , right parenthesis left parenthesis, "b" minus "a" , right parenthesis Over 2 "n" , EndFraction , right parenthesis times StartFraction, left parenthesis, "b" minus "a" , right parenthesis Over "n" , EndFractionSum=n∑i=1fa+2i−1b−a2n·b−an
