1、计算100-99 = 1，两边除以100， 1 - 0.99 = 0.01
2、任何数字等于自身乘以1, 39.4 = 39.4 x 1
3、表达式39.4-39.4×0.99，合并同类项
39.4-39.4×0.99
= 39.4 x 1 - 39.4 x 0.99
= 39.4 x (1-0.99)
4、39.4-39.4×0.99
= 39.4 x 1 - 39.4 x 0.99
= 39.4 x (1-0.99)
= 39.4 x 0.01
= 0.394
5、乘法分配律
A - B x C
= B x D - B x C
= B x (D - C)
= B x E
= F
