7.8x9.9 7.8的简便方法

Question

如题所述

Answer 1

7.8x9.9 +7.8
=7.8×（10-0.1）+7.8
=7.8×10-7.8×0.1+7.8
=78+7.8-0.78
=85.8-0.78
=85.02