二次根式是初中代数的重要内容之一 ,也是进一步学习其他数学知识的基础 .二次根式的化简和求值 ,是各级数学竞赛的热点 .一、基础知识1 .二次根式的性质 :ａ2 =|ａ|=ａ(ａ≥ 0 ) ,-ａ(ａ 0 ) .2 .共轭根式 .若ｘ =ａ +ｂ ,ｙ =ａ -ｂ ,则ｘ、ｙ是一对共轭根式 .于是有ｘ +ｙ The secondary root formula is one of the important contents of the junior high school algebra, and it is also the basis for further study of other mathematical knowledge. The simplification and evaluation of the secondary root formula is a hot topic of mathematical contests at all levels. I. Basic knowledge 1. The nature of secondary roots :a2 =|a|=a(a≥0),-a(a0) .2. Conjugate type. If x =a +b,y =a -b, then x and y are a pair of conjugate roots. So there is x + y