a必等于a,
若a + b = a c,a + 2 b = a c c,
解得b = 0,c = 1,
A等于{a,a,a},不满足集合中元素的互异性.舍去.
若a + 2 b = a c,a + b = a c c,
解得b = -(3 a)/4,c = -1/2,
另外一组解b = 0,c = 1,舍去.