脱式计算 17分之6减(17分之8减13分之11) 8分之9加6分之5加6分之7减8分之1

17分之6减(17分之8减13分之11)
=6/17-8/17+11/13
=11/13-2/17
=(187-26)/221
=161/221

8分之9加6分之5加6分之7减8分之1
=9/8+5/6+6/7-1/8
=1+35/42+36/42
=1+71/42
=2又42分之29

5/6=1/2+1/3,7/12=1/3+1/4,同理可得后面的9/20,11/30,13/42,15/56,17/72.所以原式=1-(1/2+1/3)+1/3+1/4……-(1/6+1/7)+1/7+1/8-(1/8+1/9)=7/18