Vectors and parameters

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For this question, part (e)(ii) I don't understand how the answer could be so simple. I tried a very long-winded way to prove that it was, but the answer simply says as the parameter t >0, therefore D would not lie between A and B?? 

Does anyone understand the logic behind this?

I have also attached the mark scheme. 

Any comments would be greatly appreciated! Thanks! 

[thecsstudent]

I think the answer is t<0 because to find the equation of the line with direction the vector AB you write it as l = A + tAB, where A is the point and AB is the vector. Which means that a point on the line that comes after A will have the coordinates of A PLUS tAB. However, since t is negative the point D is before A in the line and therefore is not on AB because it comes before A. I don't know if that makes sense. 

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1 hour ago, thecsstudent said:

I think the answer is t<0 because to find the equation of the line with direction the vector AB you write it as l = A + tAB, where A is the point and AB is the vector. Which means that a point on the line that comes after A will have the coordinates of A PLUS tAB. However, since t is negative the point D is before A in the line and therefore is not on AB because it comes before A. I don't know if that makes sense. 

 

Thx! Makes a lot of sense!