3/4 +1/4-3/4 +1/4=(3/4-3/4)+(1/4+1/4)=0+1/2=1/2或者:3/4 +1/4-3/4 +1/4=(3+1-3+1)/4=2/4=0.5
(3/4-3/4)+(1/4+1/4)=1/2
=(3+1-3+1)/4=1/2
