x=1/2代入方程2x-m/4-1/2=x-m/3:
1-m/4-1/2=1/2-m/3
m/3-m/4=0
m=0
1/4(-4m^2+2m-8)-(1/2m-1)
=1/4*(-8)-(-1)
=-2+1
=-1