Vectors C4

I am so so so confused about vectors, I know the basics (I is x axis, j is y axis etc) because I have been reading my revision guide for a million times but when it comes to questions, I am clueless! Please help with question 2, and is question one (3/13, -4/13, 12/13) ? For question 2 I got a=9 and b=-3 but I know it's not right :confused:

Original post by Christina Tiana

I am so so so confused about vectors, I know the basics (I is x axis, j is y axis etc) because I have been reading my revision guide for a million times but when it comes to questions, I am clueless! Please help with question 2, and is question one (3/13, -4/13, 12/13) ? For question 2 I got a=9 and b=-3 but I know it's not right :confused:

1) That's correct.

2) 'Fraid not. What's your reasoning/working?

If you're not sure how to do it, then if we call the three vectors A,B,C; if they all lie on a straight line, then AB and AC are parallel, so AB=kAC for some constant k. If you equate the coefficients you'll end up with three equations which you can solve for k,a,b.

(edited 9 years ago)

Reply 2

Christina Tiana

OP

9

I'm getting a=1, b=-11? Is that correct? That seems an awful lot of working out for 2 marks, I found AB and AC vectors from the position vectors, then I used the three eqns to find k and then A and B. K=1/3 right?

Reply 3

ghostwalker

17

Original post by Christina Tiana

I'm getting a=1, b=-11? Is that correct? That seems an awful lot of working out for 2 marks, I found AB and AC vectors from the position vectors, then I used the three eqns to find k and then A and B. K=1/3 right?

Yep, that's correct. It may seem like a lot of working, but you'll get quicker with familiarity.

Notice that the ratio of differences AB to AC are 1:3 (from the first component). If you have good mental arithmetic....

(edited 9 years ago)

Vectors C4

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