3.8×0.45-0.38×4.2=0.38×4.5-0.38×4.2=0.38×(4.5-4.2)=0.38×0.3=0.114
=3.8×（0.45-0.42）=3.8×0.03=（4-0.2）×0.03=0.12-0.006=0.194
3.8×0.45-0.38×4.2=3.8×0.45-3.8×0.42=3.8×（0.45-0.42）=3.8×0.03=0.114
