数列专项
之求和-4（一）等差等比数列前n项求和
等差数列求和公式：
<Object: word/embeddings/oleObject1.bin>
2、等比数列求和公式：
<Object: word/embeddings/oleObject2.bin>
（二）非等差等比数列前n项求和
⑴错位相减法
数列<Object: word/embeddings/oleObject3.bin>
为等差数列，数列
<Object: word/embeddings/oleObject4.bin>
为等比数列，则数列
<Object: word/embeddings/oleObject5.bin>
的求和就要采用此法.
②将数列
<Object: word/embeddings/oleObject6.bin>
的每一项分别乘以
<Object: word/embeddings/oleObject7.bin>
的公比，然后在错位相减，进而可得到数列
<Object: word/embeddings/oleObject8.bin>
的前<Object: word/embeddings/oleObject ...