3/19-7/15+7/15 3/5+5/8+2/5 1/6+(5/9+5/6)
=3/19-(7/15-7/15)=(3/5+2/5) +5/8 =(1/6+5/6)+5/9
=3/19 =1+5/8 =1+5/9
=1又5/8(或者13/8) =1又5/9 (或者14/9)
7-5/11-6/11 3/4-3/8+1/4
=7-(5/11+6/11) =(3/4+1/4)-3/8
=7-1 =1-3/8
=6 =5/8
x-2/5-2/7=3/5x-(1/5+5/6)=21/4+1/2-x=3/8
x=3/5+2/5+2/7 x=2+1/5+5/6x=1/4+1/2-3/8
x=1又2/7 或者(9/7) x=3又1/30 (或者91/30) x=3/8
7/8+1/2-2/5 1-(1/6+4/9)
=35/40+20/40-16/40 =1-(3/18+8/18)
=39/40 =1-11/18
=7/18
答：（略） 答：（略）
4. 1/3+3/4+2/5+5/7+7/8+9/20+10/21+11/24+12/35
=(1/3+10/21)+(3/4+9/20)+(2/5+5/7+12/35)+(7/8+11/24)
=(17/21+10/21)+(15/20+9/20)+(14/35+25/35+12/35)+(21/24+11/24)
=17/21+24/20+51/35+32/24
=17/21+6/5+51/35+4/3
=(17/21+28/21)+(42/35+51/35)
=45/21+93/35
=15/7+93/35
=75/35+93/35
=168/35 (或者4.8)
9/10-(1/10+2/5)
=9/10-(1/10+4/10)
=4/10
=2/5