📚 Constant-Coefficient Homogeneous Linear Differential Equations | IB数学：常系数齐次线性微分方程的概念
Differential equations are among the most powerful tools in mathematics, describing how quantities change over time or space. This article focuses on a foundational class of differential equations: constant-coefficient homogeneous linear differential equations. These equations appear frequently in the IB Mathematics Analysis and Approaches (AA) higher-level syllabus, as well as in physics and engineering courses. We will explore their standard form, the important concept of linearity and homogeneity, and the method of solving them through the characteristic equation.
微分方程是数学中最强大的工具之一，它描述了量随时间或空间的变化规律。本文聚焦于常系数齐次线性微分方程这一基础类型。这类方程在IB数学分析与方法（AA）高级课程、物理以及工程学中频繁出现。我们将研究它们的标准形式、线性与齐次的核心概念，并通过特征方程来掌握求解方法。
1. The Standard Form | 标准形式
A second-order linear differential equation with constant coefficients can be written in the general form:
a y″ + b y′ + c y = f(x)
where a, b and c are real constants, with a ≠ 0. The function f(x) is called the forcing term or non-homogeneous term. When f(x) ≡ 0, the equation becomes homogeneous:
a y″ + b y′ + c y = 0
This is the type of equation studied in this article. The word "order" refers to the highest derivative, so a second-order equation involves y″ but no higher derivatives.
二阶常系数线性微分方程的一般形式为：
a y″ + b y′ + c y = f(x)
其中 a、b、c 为实数常数，且 a ≠ 0。函数 f(x) 称为强迫项或非齐次项。当 f(x) ≡ 0 时，方程变为齐次方程：
a y″ + b y′ + c y = 0
这正是本文要研究的方程类型。"阶"指最高阶导数，因此二阶方程涉及 y″，但不涉及更高阶导数。
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