1.(-5.6)+x=-7.5
x=-7.5+5.6
x=-1.9
2.x-(+2又1/3)=5/6
x=(2又1/3) + 5/6
x=3又1/6
3.x-(-5又1/2)=-8又1/4
x=(-5又1/2) - 8又1/4
x=-13又3/4
4.1+1/2+1/6+1/12+1/20+1/30+1/42+1/56
1+1/2+1/6+1/12+1/20+1/30+1/42+1/56
=1+(1-1/2)+(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+1-1/8
=15/8
=1又7/8