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If the sequence x1, x2, x3, …, xn, … is such that $$x_{1}$$ = 3 and $$x_{n+1}$$ = $$2_{xn}$$ – 1 for n ≥ 1, then $$x_{20}$$– $$x_{19}$$ =
A、$$2^{19}$$
B、$$2^{20}$$
C、$$2^{21}$$
D、$$2^{20}$$ – 1
E、$$2^{21}$$ – 1
答案:A
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