Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    3
    ReputationRep:
    Question:

    4) In a triangle XYZ find angle Z when YZ= 4.7cm, XZ = 10.5 cm and XY = 8.9 cm.

    So I know it's cosine so I used:

    \cos \theta = \frac{a^2 + b^2 - c^2}{2ab}

    Yet I got:

    \cos^{-1} \theta = 26.43^{\circ}

    Which is wrong; somehow I think it's because I've labelled the sides wrong or something but I didn't think that mattered - could I have some help please?
    • Community Assistant
    Offline

    19
    ReputationRep:
    (Original post by Dededex)
    Question:

    4) In a triangle XYZ find angle Z when YZ= 4.7cm, XZ = 10.5 cm and XY = 8.9 cm.

    So I know it's cosine so I used:

    \cos \theta = \frac{a^2 + b^2 - c^2}{2ab}

    Yet I got:

    \cos^{-1} \theta = 26.43^{\circ}

    Which is wrong; somehow I think it's because I've labelled the sides wrong or something but I didn't think that mattered - could I have some help please?
    Draw a triangle and label the vertices X Y Z. Now label the sides. Call the angle at Z theta and call the side opposite this vertex c. It doesn't matter which of the remaning sides you call a and which one you call b.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by Mr M)
    Draw a triangle and label the vertices X Y Z. Now label the sides. Call the angle at Z theta and call the side opposite this vertex c. It doesn't matter which of the remaning sides you call a and which one you call b.
    Oh right - how come the one opposite the angle has to be c? Is that just the general rule? Or does that only apply to this specific question?
    • Community Assistant
    Offline

    19
    ReputationRep:
    (Original post by Dededex)
    Oh right - how come the one opposite the angle has to be c? Is that just the general rule? Or does that only apply to this specific question?
    If you use the Cosine Rule in the form you have given it, c is always facing the angle you are finding and a and b are the sides that meet to form the angle.
    • Community Assistant
    Offline

    19
    ReputationRep:
    mathsisfun to the rescue.

    http://www.mathsisfun.com/algebra/trig-cosine-law.html
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by Mr M)
    If you use the Cosine Rule in the form you have given it, c is always facing the angle you are finding and a and b are the sides that meet to form the angle.
    Oh right thanks alot Mr M.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What newspaper do you read/prefer?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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