①8.7-3.8×3.5÷1.9
=8.7-3.5*2
=1.7
②21/8 ×9.25+59/8 ×37/4
=9.25*10
=92.5
③3×(2/15 +1/12 )-2/5
=2/5+1/4-2/5
=1/4
④1/2 +1/6 +1/12 +1/20 +1/30 +1/42 +1/56 +1/72 +1/90
=1/2+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1-1/10
=9/10