高中向量题一道紧急求解在线等!

OP=(2,1),OM=(a,b)
O,P,M is on the same line,then
OP//OM
2/a=1/b
a=2b ---(1)
MP.MA = (OP-OM).(OA-OM)
=(2-a,1-b).(1-a,7-b)
=(2-a)(1-a) +(1-b)(7-b)
=(2-2b)(1-2b)+(1-b)(7-b)
=(1-b)(2-4b+7-b)
=(1-b)(9-5b)
=5b^2-14b+9 > 0 (MP与MA与的夹角为钝角)
MA·MB =(OA-OM) .(OB-OM)
=(1-a,7-b).(5-a,1-b)
=(1-a)(5-a) + (7-b)(1-b)
=(1-2b)(5-2b) + (7-b)(1-b)
=5-12b+4b^2 + 7-8b+b^2
= 5b^2 -20b +12
= (5b^2-14b+9) -7b +3
> -7b+3