圆方程化为:
x²-mx+m²/4+y²+y+1/4=1/4+m²/4
(x-m/2)²+(y+1/2)²=1/4+m²/4
所以圆心为(m/2,-1/2)
直线平分圆,则直线必过圆心
所以-1/2=m/2 + 1
m=-3