条件分式求值问题是初中数学中的重要内容 ,这类题形式多样 ,解法灵活多变 ,需要根据题设条件的数 (式 )值结构特征 ,巧妙地将已知与未知沟通起来 ,实现已知向未知的转化 .现介绍几种常用方法 .一、直接代入例 1　已知 xy +z=a,yz +x =b,zx +y=c,且x+y+z≠ 0 ,试求 a1 +a+b1 +b The problem of conditional fractional evaluation is an important part of junior middle school mathematics. There are various forms of such questions, and the solution methods are flexible and varied. It is necessary to skillfully communicate the known and the unknown according to the structural characteristics of the number (formula) values ​​of the conditions. Known conversion to unknown. Now introduce several common methods. First, directly into Example 1 Known xy + z = a, yz + x = b, zx + y = c, and x + y + z ≠ 0, try Find a1 +a+b1 +b