a=x+3/4,b=2x+1/3,b＜7/3＜2a
2x+1/3＜7/3＜2(x+3/4)
2x+1/3＜7/3且7/3＜2x+3/2
2x＜6/3且5/6＜2x
x＜1且5/12＜x
5/12＜x＜1
5/12+3/4＜x+3/4＜1+3/4
7/6＜x+3/4＜7/4
7/6＜a＜7/4