已知点O(0,0),A(1,2),B(4,5)及向量OP=向量AB+入向量OA(1)t为何值时,P在y轴上（2）t为何值时,P在第四象限

OP = AB +tOA
= (3,3) + t(1,2)
=( 3+t,3+2t)
P at y axis
=> 3+t =0
t=-3
(2)
P at 4th quad.
3+t >0 and 3+2t t >-3 and t < -3/2
ie -3