定义一种新运算a*b=a+b/a-b 计算-2*(-6*3)的值
问题描述：
定义一种新运算a*b=a+b/a-b 计算-2*(-6*3)的值
答
a*b=(a+b)/(a-b)
-6*3=(-6+3)/(-6-3)
=(-3)/(-9)
=1/3
-2*(-6*3)=-2*(1/3)
=(-2+1/3)/(-2-1/3)
=(-5/3)/(-7/3)
=5/7