From 8691e3a70b8352e165b49e20df60afc0a23ff2c2 Mon Sep 17 00:00:00 2001
From: Zhao Xin <7176466@qq.com>
Date: Wed, 7 Sep 2022 21:31:46 +0800
Subject: [PATCH] gitpush

---
 问题/等差数列.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/问题/等差数列.md b/问题/等差数列.md
index ddd317f..4df44b3 100644
--- a/问题/等差数列.md
+++ b/问题/等差数列.md
@@ -10,3 +10,7 @@
 $$ a_n = a + (n-1)d \tag{等差数列通项公式} $$
 
 换句话说，任意一个等差数列 $\{a_{n}\}$ 都可以写成：$\{\,a,\,a+d,\,a+2d,\,\cdots,\,a+(n-1)d\,\}$
+
+由等差数列通项公式可以得到以下公式：
+
+$$ n = \dfrac{a_末-a}{d} + 1 \tag{已知首项尾项及公差求项数}$$