|x-2|与（y+1/2）²互为相反数
|x-2|+（y+1/2）²=0
x=2 y=-1/2
x²y-3xy²-1/3（3x²y-2 xy²）
=x²y-3xy²-3x²y+2 xy²/3
=-2x²y-7 xy²/3
=-2*2^2*(-1/2)-7*2*(-1/2)^2/3
=4-7/6
=17/6