a方x-a方y分之x方-y方乘（x+y)方分之ax+ay
=a²(x-y)分之(x+y)(x-y) × (x+y)²分之a(x+y)
=a²分之(x+y) × (x+y)分之a
=a分之1 = 5
则,a=5分之1