Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    12
    ReputationRep:
    Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
    Attached Images
      
    Offline

    20
    ReputationRep:
    if 5a/4 = 5b/-13 = c

    let c = 1 without loss of generality

    5a/4 = 1

    5b/-13 = 1

    a = 4/5

    b = -13/5

    c = 1

    these are the three components of the vector n

    the direction cosines can be read off from the unit vector .... the magnitude of n is (√210)/5

    so the first direction cosine is ( 4/5 ) / (√210)/5

    etc
    Offline

    11
    ReputationRep:
    (Original post by pippabethan)
    Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
    I get (\frac{-4}{\sqrt{210}}, \frac{13}{\sqrt{210}}, \frac{-5}{\sqrt{210}}) by taking the cross product.

    [And having thought about the OP's working, I think that the cross product approach is quicker]
    Offline

    20
    ReputationRep:
    (Original post by atsruser)
    I get (\frac{-4}{\sqrt{210}}, \frac{13}{\sqrt{210}}, \frac{-5}{\sqrt{210}}) by taking the cross product.

    [And having thought about the OP's working, I think that the cross product approach is quicker]
    i was not sure if they had to know about the x product :beard:
    Offline

    11
    ReputationRep:
    (Original post by pippabethan)
    Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
    As you can see from the bear's working, the components of your vector are (a,b,c) - you are:

    a) working with components (5a/4, -13b/5, c)
    b) failing to divide by the length of (a,b,c)
    Offline

    11
    ReputationRep:
    (Original post by the bear)
    i was not sure if they had to know about the x product :beard:
    They do. See top of the picture. My Mycroft Holmes-like power of observation saves the day yet again. They are also using the old fashioned wedge notation, so she is probably at somewhere with time mellowed wood panelling on the walls. They tend to go together.
    • Thread Starter
    Offline

    12
    ReputationRep:
    Thank you everyone!! I've now corrected it, and done it the cross product way (much quicker, but didn't think of it haha)
    Offline

    11
    ReputationRep:
    (Original post by pippabethan)
    Thank you everyone!! I've now corrected it, and done it the cross product way (much quicker, but didn't think of it haha)
    It never hurts to be aware of multiple methods though. Is this in fact A level stuff? Which syllabus has direction cosines?
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by atsruser)
    It never hurts to be aware of multiple methods though. Is this in fact A level stuff? Which syllabus has direction cosines?
    Technically I'm at uni, but I'm doing a science course, and I figured the stuff would probably be simple enough to be in fp modules!
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 13, 2017
Poll
“Yanny” or “Laurel”
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.