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

    16
    ReputationRep:
    Today I have been doing a past GCSE Maths paper and one of the questions has really stumped me. Not because I don't know what to do, but I can't seem to get the right answer!

    In the question I have to calculate a length, AB, which is part of a triangle ABC. Effectively it is side c as it's opposite to angle C which is 75. The other information I have is that the other side lengths are 5 and 8cm. (sorry I can't provide it visually, can't find it online)

    I thought the formula should go like this:
    c^2=b^2+a^2-2ab cos C

    And therefore this:
    c^2=25+64-2x8x5 cos 75

    When I input it into my calculator I get c^2 = 2.33cm (3 s.f.)
    This can't be right, can it? On the diagram it is shown significantly larger than the side with length 5cm and I know they are not drawn "accurately" but still. Please help, this is driving me mad!! Am I inputting it wrongly? Many thanks in advance.
    • PS Helper
    Offline

    14
    PS Helper
    How did you get that? What did you type into your calculator? c^2=25+64-2\times8\times5 \cos 75^{\circ} is correct, and the RHS certainly doesn't evaluate to 2.33.
    Offline

    17
    ReputationRep:
    Are you sure you typed it in your calculator correctly? I get a different answer, c^2=68.29 so c=8.26 which fits.
    • PS Helper
    Offline

    14
    PS Helper
    (Original post by ghostwalker)
    Is your calculator in radians mode (incorrectly)?
    Even if it was it wouldn't give anything near 2.33.
    • Thread Starter
    Offline

    16
    ReputationRep:
    (Original post by nuodai)
    How did you get that? What did you type into your calculator? c^2=25+64-2\times8\times5 \cos 75^{\circ} is correct, and the RHS certainly doesn't evaluate to 2.33.
    Thanks for the confirmation that my working is correct. I think I am typing it in wrong, as I've tried this on two different calculators now. I typed it in all in one go (as in, with no brackets or anything) up until the cos 75 part. This gave me the answer 9, which multiplied by cos 75 gives me 2.33. I multiplied it by cos by inputting the multiplication sign, then the number 75, then cos. I am extremely confused :confused:

    edit: it is not in radians mode
    Offline

    17
    ReputationRep:
    You need to do 89-(80*cos 75). Make sure you process the bracket first, otherwise you will get 2.33, not 8.26
    • Thread Starter
    Offline

    16
    ReputationRep:
    (Original post by jameswhughes)
    You need to do 89-(80*cos 75). Make sure you process the bracket first, otherwise you will get 2.33, not 8.26
    Thank you so much, I knew there was something I was doing wrong!! This is a huge help, thanks.
    • PS Helper
    Offline

    14
    PS Helper
    (Original post by Where'sPerry?)
    Thanks for the confirmation that my working is correct. I think I am typing it in wrong, as I've tried this on two different calculators now. I typed it in all in one go (as in, with no brackets or anything) up until the cos 75 part. This gave me the answer 9, which multiplied by cos 75 gives me 2.33. I multiplied it by cos by inputting the multiplication sign, then the number 75, then cos. I am extremely confused :confused:

    edit: it is not in radians mode
    Ah I see. What you did was work out a^2+b^2-2ab and then multiply it by \cos C, so that gives (a^2+b^2-2ab) \times \cos C, which is equal to a^2 \cos C + b^2 \cos C - 2ab \cos C (by expanding the bracket). What you actually want is a^2+b^2 - (2 ab \cos C).

    What you need to do is type it all in in one go, including the \cos 75 bit. Alternatively, work out -2 \times 5 \times 8, then multiply that by \cos 75, and then add on 5^2+8^2.

    [EDIT: Basically what I'm saying is that you have to follow BIDMAS.]
    Offline

    1
    ReputationRep:
    (Original post by Where'sPerry?)
    Today I have been doing a past GCSE Maths paper and one of the questions has really stumped me. Not because I don't know what to do, but I can't seem to get the right answer!

    In the question I have to calculate a length, AB, which is part of a triangle ABC. Effectively it is side c as it's opposite to angle C which is 75. The other information I have is that the other side lengths are 5 and 8cm. (sorry I can't provide it visually, can't find it online)

    I thought the formula should go like this:
    c^2=b^2+a^2-2ab cos C

    And therefore this:
    c^2=25+64-2x8x5 cos 75

    When I input it into my calculator I get c^2 = 2.33cm (3 s.f.)
    This can't be right, can it? On the diagram it is shown significantly larger than the side with length 5cm and I know they are not drawn "accurately" but still. Please help, this is driving me mad!! Am I inputting it wrongly? Many thanks in advance.
    Put the whole thing in at once. I had this problem before.
 
 
 
Poll
Do you agree with the PM's proposal to cut tuition fees for some courses?
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.