直线l过点(0,1/2)=> y-1/2=k(x-0) =>y=kx+1/2直线l与抛物线y=1/2x^2相交于A.B两点=>A(x1,1/2 x1^2)B(x2,1/2 x2^2)1/2 x1^2 = kx1+1/21/2 x2^2 = kx2+1/2=>x1^2 = 2kx1+1x2^2 = ...