設直線l:y=k(x-1/2)
代入y^2=2x,得:k^2x^2-(k^2+2)x+k^2/4=0
設A(x1,y1),B(x2,y2)
則x1x2=1/4 x1+x2=(k^2+2)/k^2
y1y2=k^2(x1-1/2)(x2-1/2)=k^2[x1x2-1/2(x1+x2)+1/4]
=k^2[1/4-(k^2+2)/2k^2+1/4]
=1/2k^2(1-1-2/k^2)=-1
x1x2+y1y2=1/4-1=-3/4
當斜率不存在時,同理可得:x1x2+y1y2=-3/4
所以值為-3/4