The Student Room Group

Vectors.. again.

I suspect I went wrong somewhere/am stuck on part (b)

Sigh, I need some sleep.
Reply 1
Original post by edothero
I suspect I went wrong somewhere/am stuck on part (b)

Sigh, I need some sleep.


my approach would be different

Midpoint of AB is (2, -1/2, 1/2)

M must also be the midpoint of OC

Since O(0,0,0), C(4,-1,1)

any good?


EDIT: Rubbish on my part. I read OACB instead of OABC

I can also do one with "vector vectors"
(edited 9 years ago)
Reply 2
Original post by edothero
I suspect I went wrong somewhere/am stuck on part (b)

Sigh, I need some sleep.


sorry I misread part (b)

Do you know dot product, cross product or both?
Reply 3
Original post by TeeEm
sorry I misread part (b)

Do you know dot product, cross product or both?


just the dot product

a.b=abcosθa.b=\left | a \right |\left | b \right |cos\theta
Reply 4
Original post by edothero
just the dot product

a.b=abcosθa.b=\left | a \right |\left | b \right |cos\theta


numbers are bad

are you on a calculator course or non calculator?
Reply 5
Original post by TeeEm
numbers are bad

are you on a calculator course or non calculator?


It's Edexcel C4; calculator.
Reply 6
Original post by edothero
It's Edexcel C4; calculator.


then I doubt if they want exact answer

Use the OA and AB to find angle OAB (dot product)

then trigonometric area of the triangle x2
Reply 7
I got around 20.7
Reply 8
Original post by TeeEm
then I doubt if they want exact answer

Use the OA and AB to find angle OAB (dot product)

then trigonometric area of the triangle x2


Thanks again!
Reply 9
Original post by edothero
Thanks again!


no problem
Original post by edothero
I suspect I went wrong somewhere/am stuck on part (b)

Sigh, I need some sleep.

I managed to get through A2 maths without being able to do the most basic vectors questions so i'm no use, good luck!

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