The Student Room Group

Help with graph questions please.

In the diagram,

the points A, B and C lie on the straight line y = 2x 1

The coordinates of A are (2, 3).
The coordinates of B are (5, 9).
Given that AC = 3AB, find the coordinates of C


I did 5-2 =3 and then 9-3 = 6 and then multiplied them by 3 to get 9,18, however, the mark scheme says the answer is 11,21 and I do not understand how they worked it out.

Thank you.
You have found 3AB (9,18) but this isn't the coordinate of C. Add 3AB to A coordinate and see what you get.
Original post by TheVirtualPhoton
You have found 3AB (9,18) but this isn't the coordinate of C. Add 3AB to A coordinate and see what you get.


Thanks, I got 11,21, but why do you have to add coordinate A to 3AB? What does the 9,18 stand for?
9,18 is 3 times the length AB, and acts as a vector to move from coordinate A to coordinate C.
If you think of getting from A to B, you have to go 3 units along the x direction, then 6 units up the y direction. To get to C you have to do this 3 times, so go 9 units along x and 18 units up y to get from A to C.
Original post by TheVirtualPhoton
9,18 is 3 times the length AB, and acts as a vector to move from coordinate A to coordinate C.
If you think of getting from A to B, you have to go 3 units along the x direction, then 6 units up the y direction. To get to C you have to do this 3 times, so go 9 units along x and 18 units up y to get from A to C.


ohhhhhhhhhhhhh so becuase A is 2,3 and not 0,0 we need to add it to 9,18 , thank youuuu I get it now.

Quick Reply

Latest