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、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
