an = a1q^(n-1)
Sn = a1( 1- q^n)/(1-q)
lim(n->∞) Sn = 1/2
a1/(1-q) = 1/2
a1 = (1/2)(1-q)
0
等比数列an的前n项和为sn.且sn的极限=1/2.求a1范围
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