X/(Y+Z)+Y/(X+Z)+Z/(X+Y)=1设x/(y+z)=a,y/(x+z)=b,z/(x+y)=c∴a+b+c=1 x=a(y+z) y=b(x+z) z=c(x+y)相加：x+x+z=(b+c)x+(a+c)y+(a+b)z∴[1-(b+c)]x+[1-(a+c)]y+[1-(a+b)]z=0∴ax+by+cz=0即X^2/(Y+Z)+Y^2/(X+Z)+Z^2/(X+Y)=0
