Q(x,y)
PQ中点M(x-4/2,y+4/2)
M到x－2y－7=0距离
d=|x-4/2-y-4-7|/根号(1+4)=|x-2y-19|/2根号5
设k=x-2y
x=k+2y
(k2+4ky+4y2)/64+y2/9=1
25/144y2+1/16ky+k2/64-1=0
△=1/256k2-4*25/144*(k2/64-1)
=-1/144k2+25/36>=0
-10