本文给出关于变元x的线性分式(ax+b)/(cx+d)恒为定值的充要条件的一个应用——求解一类条件二元分式和为定值问题.由于(ax+b)/(cx+d)=a/c+(bc-ad)/c(cx+d),故得如下结论:设x为实数变量,a,b,c,d为常数,且c≠0,则关于x的线性分式(ax+b)/(cx+d)恒为定值a/c的充要条件为ad=bc. In this paper, we give an application of the necessary and sufficient conditions for the constant value of the linear fraction (ax + b) / (cx + d) of the argument x to solve a class of conditional binary fractions and the problem of fixed value. (ax + b) / (cx + d) = a / c + (bc- ad) / c (cx + d), we conclude that x is a real variable and a, b, c and d are constants, c ≠ 0, then the necessary and sufficient conditions for the linear fractional (ax + b) / (cx + d) constant for a to be a / c are ad = bc.