# Help with vectors...

• Dec 7th 2008, 08:02 AM
struck
Help with vectors...
I have tried solving this again and again but I get wrong answer. (Headbang)

Points A and B have coordinates (2,7) and (-3,-3) respectively. Use a vector method to find the coordinates of C and D, where

(a) C is the point such that AC = 3AB (AC and AB have that arrow for vector notation)

(b) D is the point such that AD = 3/5 AB ..

I only need (a) and I hopefully that will help me understand it.

Thanks
• Dec 7th 2008, 08:16 AM
running-gag
Quote:

Originally Posted by struck
I have tried solving this again and again but I get wrong answer. (Headbang)

Points A and B have coordinates (2,7) and (-3,-3) respectively. Use a vector method to find the coordinates of C and D, where

(a) C is the point such that AC = 3AB (AC and AB have that arrow for vector notation)

(b) D is the point such that AD = 3/5 AB ..

I only need (a) and I hopefully that will help me understand it.

Thanks

Hi

A(2,7) and B(-3,-3)
Therefore vector AB(-3-2,-3-7)
AB(-5,-10)

C is the point such that AC = 3AB
AC(x-2,y-7)
3AB(-15,-30)

Therefore
x-2=-15 and y-7=-30
x=-13 and y=-23
• Dec 7th 2008, 08:17 AM
Mentia
so the vector AB is (-5, -10)

If AC is 3AB then AC is (-15, -30)

start at A and go down 30 left 15 and you get C = (2-15, 7-30) = (-13, -23)
• Dec 7th 2008, 08:23 AM
Skalkaz
Dear struck,

mark the origo O in koordinates (0, 0)
So
OA = (2,7)
OB = (-3,-3)
AB = OB - OA = (-3-2, -3-7) = (-5,-10)
AC = 3*AB = (3*(-5),3*(-10)) = (-15,-30)

If the question is OC than
since AC = OC - OA --> OC = AC + OA = (-15+2,-30+7) = (-13,-23)
The point C is (-13,-23).