怎么计算等差数列中的项数

有时我们在数学中会遇到这样的问题，需要找出等差数列中的项数。这个问题并不难，按照以下方法就能求出。

步骤

1. {"smallUrl":"https:\/\/www.zenmeban.com\/images_en\/thumb\/d\/d1\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-1.jpg\/v4-460px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-1.jpg","bigUrl":"https:\/\/www.zenmeban.com\/images\/thumb\/d\/d1\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-1.jpg\/v4-728px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-1.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><\/div>"} 1 找出公差。题目中可能会直接给出公差，或者在数列的首或尾给出两个连续的数字。公差用d表示。^[1]
2. {"smallUrl":"https:\/\/www.zenmeban.com\/images_en\/thumb\/a\/ad\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-2.jpg\/v4-460px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-2.jpg","bigUrl":"https:\/\/www.zenmeban.com\/images\/thumb\/a\/ad\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-2.jpg\/v4-728px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><\/div>"} 2 找出数列的首项和末项。要求出等差数列的项数必须要知道首项和末项。找出后记下来。首项用A表示，末项用L表示。^[2]
3. {"smallUrl":"https:\/\/www.zenmeban.com\/images_en\/thumb\/3\/38\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-3.jpg\/v4-460px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-3.jpg","bigUrl":"https:\/\/www.zenmeban.com\/images\/thumb\/3\/38\/Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-3.jpg\/v4-728px-Find-a-Number-of-Terms-in-an-Arithmetic-Sequence-Step-3.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><\/div>"} 3 用下列公式计算项数：项数用n表示。公式为：

   n = (L-A)/d + 1

   公式很简单。用末项（L）与首项（A）之差除以公差（d），再加1。^[3]

小提示

• 末项与首项之差总是能被公差整除。

警告

• 不要将首末项之差与公差混淆。