# Thread: vectors in 2D space again

1. ## vectors in 2D space again

question: With respect to an origin, O, the points A, B and C have position vectors (2i+5j)m, (-i+2j)m and (7i+15j)m respectively.
(a) Express vector BA, in terms of i and j.
(b) Given that P is the point on BA, between B and A, such that BP/PA =1/2
Write down vector BP, in terms of i and j
(c) Hence, or otherwise, find vector OP, in terms of i and j.

my ans:
(a) -3m(i+j)
(b) -m(i+j)
(c) 3mj
book's ans:
(a) 3i+3j
(b) i+j
(c) 3j

i have no idea where all the 'm' goes

2. Originally Posted by afeasfaerw23231233
question: With respect to an origin, O, the points A, B and C have position vectors (2i+5j)m, (-i+2j)m and (7i+15j)m respectively.
(a) Express vector BA, in terms of i and j.
(b) Given that P is the point on BA, between B and A, such that BP/PA =1/2
Write down vector BP, in terms of i and j
(c) Hence, or otherwise, find vector OP, in terms of i and j.

my ans:
(a) -3m(i+j)
(b) -m(i+j)
(c) 3mj
book's ans:
(a) 3i+3j
(b) i+j
(c) 3j

i have no idea where all the 'm' goes
The "m" is probably just a distance unit: a meter.

You have the correct answers for a) and b), the book didn't print (I presume) the negative sign.

-Dan

3. oops, thanks!

$\overrightarrow{OA}=2\overrightarrow{i}+5\overrigh tarrow{j}, \ \overrightarrow{OB}=-\overrightarrow{i}+2\overrightarrow{j}$
Then $\overrightarrow{BA}=\overrightarrow{OA}-\overrightarrow{OB}=2\overrightarrow{i}+5\overrigh tarrow{j}+\overrightarrow{i}-2\overrightarrow{j}=3\overrightarrow{i}+3\overrigh tarrow{j}$