首先考虑k=0的情况,显然k=0满足题意；
当k≠0时,设点M（x1,y1),N（x2,y2）,
则y1=x1^2;y2=x2^2;
(y1-y2)/(x1-x2)=1/k;
(y1+y2)/2=-k(x1+x2)/2+9/2;
又y1-y2=x1^2-x2^2=(x1+x2)*(x1-x2)
即(y1-y2)/(x1-x2)=x1+x2
所以k=1/(x1+x2)
则(y1+y2)/2=-k(x1+x2)/2+9/2=4
y1+y2=x1^2+x2^2=8
(x1+x2)^2=x1^2+x2^2+2*x1x2