大家知道 ,式子 a (a≥ 0 )叫做二次根式。理解这个概念 ,要注意以下几点 :1 .表示非负数 a的算术平方根的式子叫做二次根式。2 .在实数范围内 ,负数没有平方根 ,所以当 a0时 ,a没有意义 ,如 - 1、 - 2不能叫做二次根式 ,而2 x- 4只有当 2 x- 4≥ 0 ,即 x≥ 2时才是二次根式。 As we all know, the formula a (a≥ 0) is called the secondary root formula. To understand this concept, pay attention to the following points: 1. The expression that represents the square root of the non-negative number a is called a quadratic root. 2. In the range of real numbers, the negative number has no square root, so when a0, a does not have meaning, such as -1, -2 can not be called quadratic root, and 2 x - 4 only when 2 x - 4 ≥ 0, that is x ≥ 2 It is a secondary root.