【答案解析】
1.觀察算式發(fā)現(xiàn):減數(shù)是個接近整數(shù)10,可以將減數(shù)拆成-10-3,然后用52-10=42,最后42-3=39.
52-13
=52-10-3
=42-3
=39
2.25-18
=25-20+2
=5+2
=7
3.31-19
=31-20+1
=11+1
=12