原式=(1/2-8/3)/(3/4)=(1/2×12-8/3×12)/(3/4×12)=(6-32)/9=-26/9
原式=(9/13)/(26/15)*6/5=9/13×15/26×6/5=81/169
原式=17×3/4+23×3/4=3/4×（17+23）=3/4×40=30