求和符号：
n
例如 S = ∑ xi = x1 + x2 + .+ xn
i =1
例如：x1 = 1,x2 = 2,...,x10 = 10
那么：
10
S = ∑ xi = x5 + x6 + x7 + x8 + x9 + x10 = (5+6+...+10) = 45 .
i =5