题目内容
{an}是无穷数列,已知an是二项式(1+2x)n(n∈N*)的展开式各项系数的和,记
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【答案】分析:由题意可得an=3n,利用等比数列的前n项和公式求出Pn,再利用数列极限的运算法则求出结果.
解答:解:由题意可得an=3n,∴
=
+
+
+…+
=
=
,
∴
=
=
.
故答案为:
.
点评:本题考查二项式定理,等比数列的前n项和公式,求数列极限的方法,求出Pn=
,是解题的难点和关键.
解答:解:由题意可得an=3n,∴

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∴

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故答案为:

点评:本题考查二项式定理,等比数列的前n项和公式,求数列极限的方法,求出Pn=


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