简便计算:1/30+1/42+1/56+1/72=
问题描述:
简便计算:1/30+1/42+1/56+1/72=
急
答
=1/5×6+1/6×7+1/7×8-1/8×9
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45