Re: [機經] 幾題機經

[catspace (柏拉圖的永恆...)]

: 272.數列x1，x2，x3.。。。xn。x1=3.已知xn+1=2xn-1，求x20-x19
: 沒有給答案, 可是我覺得題目如果沒有給X2的實際數字這提不能算??

X20-X19

= [2(X19)-1] - X19

= X19 - 1

= [2(X18)-1] - 1

= 2(X18) - 2

= 2[2(X17)-1] - 2

= 4(X17) - 4

= 2^2(X17) - 2^2

..............

= 2^18(X1) - 2^18

= 2^18(X1-1)

= 2^18*2

= 2^19 #

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