54.9*0.38
=54.9*(0.4-0.02)
=54.9*0.4-54.9*0.02
=21.96-1.908
=20.862
小数乘法的运算法则:
1、先按照整数乘法的法则求出积;
2、再看被乘数和乘数一共有几位小数,就从积的右边起数出几位,点上小数点;
3、如果小数的末尾出现0时,根据小数的基本性质,把小数末尾的0划去。
54.9*0.38
=54.9*(0.4-0.02)
=54.9*0.4-54.9*0.02
=21.96-1.908
=20.862
54.9*0.38=(55-0.1)*0.38=50*0.38+5*0.38-0.1*0.38=19+1.9-0.038=20.9-0.038=20.862
交换律来算