令G(x)= 1 /(1 – x) 2 = ,其中| x | <1.什么是g(i)?
(A)我
(B)我+1
(C) 2i
(D) 2我答案: (B)
说明: B是正确的选项。让我们把价值观
S = 1 + 2x + 3x2 + 4x3 + ..........
Sx = x + 2x2 + 3x3 + ..........
S - Sx = 1 + x + x2 + x3 + ....
S - Sx = 1/(1 - x) [sum of infinite GP series with ratio < 1 is a/(1-r)]
S = 1/(1 - x)2
这个问题的测验