5.27-2.1+1.73-1.9=5.27+1.73-2.1-1.9=（5.27+1.73）-（2.1+1.9）=7-4=3
5.27-2.1+1.73-1.9=（5.27+1.73）-(2.1+1.9)=7-4=3
=(5.27+1.73)-(2.1+1.9)=7-4=3
