计算；
5/8+3/8-5/8+3/8= (5/8-5/8)+(3/8+3/8)=3/4
4.28+(5.72-3.99)= 4.28+5.72-3.99=10-3.99=6.01
1/7* (42+7/11)=1/7×42+1/7×7/11=6+1/11=6 1/11
8.1*1/6+8.1*5/6=8.1(1/6 +5/6)=8.1
1/2*1/3除1/2*1/3=1/3×1/3=1/9