更正：应该是当QA*QB取最小值时,求Q的坐标
不妨设Q点的坐标是(2m,m),OQ=(2m,m)
则QA=OA-OQ=(1-2m,7-m)
QB=OB-OQ=(5-2m,1-m)
QA*QB
=(1-2m)(5-2m)+(7-m)(1-m)
=(5-12m+4m²)+(7-8m+m²)
=5m²-20m+12
=5(m-2)²-8
所以当m=2时,QA*QB取最小值,此时Q(4,2),OQ=(4,2)