乘法分配律3.9×6.7+3.9+3.9×2.3=3.9x（6.7+1+2.3）=39x10=390
=3.9×(6.7+1+2.3)=3.9×10=39
3.9×（6.7+2.3+1）=39
=3.9x6.7加3.9x一加3.9x2.3
3.9x7.6+3.9x12.4
