Hi everyone. I'm having some problems with this couple examples.

Let OAB be a triangle, that is, O,A and B are not collinear. Now let R and S be the mid-points of the sides AB and OA respectively and let M be the point of intersection of the line segments OR and BS.

a)

Express the vector OS as a linear combination of OA and OB.

Is it : OS = OB - 1/2(OA) ??

b)

Express the vector OR as a linear combination of OA and OB.

Is it: OR = OB + 1/2(BA) = OB + 1/2(OA - OB) = 1/2(OA + OB)??

c)

Give the vector equation of the line through O and R in terms of OA and OB.

Is it: OR = OB + BR = OB + tBA???

d)

Give the vector equation of the line through B and S in terms of OA and OB.

e)

Express the vector OM as a scalar multiple of OR.

thanks for any help.