y=(-1/2)x^2+(2/3)x+2
y=0
x=(2±2√10)/3
A(a,0)
L:y=k(x-a)
(-1/2)x^2+(2/3)x+2=k(x-a)
3x^2+(4+6k)x-12-6ka=0
x1+x2=-(4+6k)/3=0
k=-2/3
3x^2-12-6*(-2/3)a=0
3x^2-12+4a=0
x^2=(12-4a)/3≥0
(2-2√10)/3