1.2乘以(-1又5分之4)乘以(-2.5)乘以(-9分之7)
=6/5*（-9/5）*(-5/2)*(-7/9)=
6*(-9)*(-5)*(-7)
------------------- (注意约分)=
5*5*2*9
-3*7
-----
5
= -21/5 =-4.2