S1=9/2 -13/2 -5/2 +2
S1=(9-13-5)/2+2=(-4-5)/2+2=-9/2+2=(-9+4)/2=5/2
S2=1.76-2.04+1.12+0.92-3.23
S2=-0.28+1.12+0.92-3.23=0.84+0.92-3.23=1.76-3.23=-1.47
S3=113/20 +12.25-71/10+5.65-17.9
S3=5.65+12.25-7.1+6.65-17.9
S3=25-7.1+6.65-17.9=17.9+6.65-17.9=24.55-17.9=6.65
