bettyofarim 当前离线
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Q29:
If the sequence x1, x2, x3, …, xn, … is such that x1 = 3 and xn+1 = 2xn – 1 for n ≥ 1, then x20 – x19 =A. 219B. 220C. 221D. 220 - 1E. 221 - 1我怎么都算不出答案,谢谢各位大侠
daoying 当前离线
X20 - X19 = (2 * X19 - 1) - X19 = X19 - 1= (2 * X18 - 1) - 1= 2 * (2 * X17 - 1) - 2= 2 * (2 * (2 * X16 - 1) - 1) - 2= 2 * 2 * 2 * X16 - (2 * 2 - 2 - 1) - 1= 2^18 * X1 - (2^17 - 2^16 - ... - 2^2 - 2^1 - 2^0) - 1= 2^18 * 3 - sum of(2^k) where k goes from 0 to 17 (see note) - 1= 2^18 * 3 - 2^0 * (2^18 - 1) / (2 - 1) - 1= 2^18 * 3 - 1 * 2^18 + 1 - 1= 2^18 * 2= 2^19
Note: sum of (x^k) where k goes from y to z = x^y * (x^(z - y + 1) - 1) / (x - 1)
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longxiaomeim 当前离线
自2003年开始提供 MBA 申请服务以来,保持着90% 以上的成功率,其中Top10 MBA服务成功率更是高达95%