如果把代数式中任意两个字母对换后代数式保持不变 ,则称这样的代数式为对称式 .例如 :ａ+ｂ ,ａｂ ,ａ2 +ｂ2 ,ｘ2 + ｙ2 等都是对称式 .我们知道 :只要已知ａ +ｂ的值和ａｂ的值 ,就可以求出关于ａ、ｂ的对称式的值 .例如 :当ａ+ｂ=3,ａｂ=12 时 ,ａ2 +ｂ2 =(ａ+ｂ) 2 - 2ａ If the algebraic expression of any two letters in the algebraic formula is not changed, the algebraic expression is said to be symmetric. For example: a+b, ab, a2 + b2, x2 + y2, etc. are symmetric. We know that: Knowing the value of a + b and the value of ab, we can find symmetry values ​​for a, b. For example: when a + b = 3, ab = 12, a2 ​​+ b2 = (a + b) 2 - 2a