文章目录
- 等比数列
等比数列
首项为 a 1 a_{1} a1,公比为q
第n项: a n = a 1 q n − 1 a_{n}=a_{1}q^{n-1} an=a1qn−1
前n项和: S n = a 1 + a 2 + . . . + a n = a 1 + a 1 q + . . . + a 1 q n − 1 S_{n}=a_{1}+a_{2}+...+a_{n}=a_{1}+a_{1}q+...+a_{1}q^{n-1} Sn=a1+a2+...+an=a1+a1q+...+a1qn−1
S n = { n a 1 , q = 1 a 1 ( 1 − q n ) 1 − q , q ≠ 1 S_{n}=\left\{\begin{matrix} na_{1}&,q=1\\ \frac{a_{1}(1-q^{n})}{1-q}&,q\neq 1 \end{matrix}\right. Sn={na11−qa1(1−qn),q=1,q̸=1
S ∞ = a 1 1 − q , ( ∣ q ∣ < 1 , n → ∞ ) S_{\infty}=\frac{a_{1}}{1-q},(|q|<1,n\to \infty) S∞=1−qa1,(∣q∣<1,n→∞)