(x-y)/(x+y)=3求( 3x-2y-1)/(x+y-5)
∵（x-y)/(x+y)=3,
∴可设 x-y=3a.(1)(a≠0)
x+y=a.(2)
(1)+(2)得 2x=4a,故x=2a; (2)-(1)得 2y=-2a, 故y=-a,
于是 (3x-2y-1)/(x+y-5)=(3a+2a-1)/(2a-a-5)=(5a-1)/(a-5)
当a=1时,原式=(5-1)/(1-5)=-1
因为a是x-y与x+y的公因数,其值不定,原式的值也就不可能确定.