Q:an=n⋅3n−1,求Sn.Q:a_{n}=n\cdot 3^{n-1},求S_{n}.Q:an=n⋅3n−1,求Sn.
A:Sn=(n2−14)⋅3n+14.A:S_{n}=\left( \dfrac {n}{2}-\dfrac {1}{4}\right) \cdot 3^{n}+\dfrac {1}{4}.A:Sn=(2n−41)⋅3n+41.
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提升题
an=a1+(n−1)d,bn=qn−1(q≠0,1),cn=anbn,Sn=∑i=1nci,推导Sn.a_{n}=a_{1}+\left( n-1\right) d,\\ b_{n}=q^{n-1}( q\neq 0,1),\\ c_{n}=a_{n}b_{n},\\ S_{n}=\sum ^{n}_{i=1}c_{i},\\ 推导S_{n}.an=a1+(n−1)d,bn=qn−1(q=0,1),cn=anbn,Sn=∑i=1nci,推导Sn.
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