The sequence a1, a2, … , a n, … is such that an = 2an-1 - x for all positive integers n ≥ 2 and for certain number x. If a5 = 99 and a3 = 27, what is the value of x?
3
9
18
36
45
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应该是an=2an-1-x, a5=2a4-x, a4=2a3-x
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应该是an=2an-1-x, a5=2a4-x, a4=2a3-x