b分之a-a分之b-ab分之a平方+b平方
=(a²-b²-a²-b²)/ab
=-2b²/ab
=-2b/a
∵9a的平方-4b的平方=0,
∴9a²=4b²
a=±2b/3
∴-2b/a
=-2b÷(±2b/3)
=±3