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Possible error in the mark scheme or the question papers?

The part (i) of quesiton 7 asks to show that the angle BAC is cos-1 1/3. To calculate this angle we need to do this: AC.AB = |AC|.|AB|.cos x but the mark scheme gives the vector AB WRONG!?

Question paper: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_qp_13.pdf

Mark Scheme: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_ms_13.pdf
Original post by nonipaify
The part (i) of quesiton 7 asks to show that the angle BAC is cos-1 1/3. To calculate this angle we need to do this: AC.AB = |AC|.|AB|.cos x but the mark scheme gives the vector AB WRONG!?

Question paper: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_qp_13.pdf

Mark Scheme: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_ms_13.pdf


I think you're right. Luckily, it doesn't affect the rest of the question, but the first component of AB should be -2 not 4.
Reply 2
Avatar for davros
davros
16
Original post by nonipaify
The part (i) of quesiton 7 asks to show that the angle BAC is cos-1 1/3. To calculate this angle we need to do this: AC.AB = |AC|.|AB|.cos x but the mark scheme gives the vector AB WRONG!?

Question paper: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_qp_13.pdf

Mark Scheme: http://onlineexamhelp.com/wp-content/uploads/2014/08/9709_s14_ms_13.pdf


I agree that the first coordinate of vector AB is wrong as shown. However, as it's multiplied by 0 in the dot product it makes no difference to that product.
Reply 3
Avatar for nonipaify
nonipaify
OP
3
Original post by davros
I agree that the first coordinate of vector AB is wrong as shown. However, as it's multiplied by 0 in the dot product it makes no difference to that product.


But it does make a difference when we take the modulus/magnitude of the vector AB which results in a different final value for the angle.
Reply 4
Avatar for davros
davros
16
Original post by nonipaify
But it does make a difference when we take the modulus/magnitude of the vector AB which results in a different final value for the angle.


If anything it looks like the QP might be wrong!

I don't know anyone who takes this board - it's international, right? - so not sure who can confirm this.

Sometimes corrections are issued in hardcopy at the time of the exam but not on online pdf files., Is this an official exam board download?
Reply 5
Avatar for nonipaify
nonipaify
OP
3
Original post by davros
If anything it looks like the QP might be wrong!

I don't know anyone who takes this board - it's international, right? - so not sure who can confirm this.

Sometimes corrections are issued in hardcopy at the time of the exam but not on online pdf files., Is this an official exam board download?


Nope its not from an official source and that might be the reason. And yes it is an international board but any ways, whats important now after confirming the error is that this question doesnt make me need to worry about my ability to attempt vector questions :smile:

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