Turn on thread page Beta

How to rearrange this equation to find x? watch

    • Thread Starter
    Offline

    13
    ReputationRep:
    Okay, so I'm doing the cosine rule in maths and I can't figure out how to rearrange this ugly formula to get the length of one of the sides of a triangle. I've inputted all of the numbers but am struggling with the rearranging. Could someone please go over it with me step by step?

    9.92 = x2+7.12 + x(14.2cos(47))

    I then minused the 7.1 squared to get this:

    9.92 - 7.12 = x2 + x(14.2cos(47))

    then i totalled the left hand side to get this:

    47.6 = x2 + x(14.2cos(47))

    but it is still really disgusting and I can't figure it out help?
    • Community Assistant
    • Very Important Poster
    Offline

    20
    ReputationRep:
    Community Assistant
    Very Important Poster
    (Original post by Bhansongal98)
    Okay, so I'm doing the cosine rule in maths and I can't figure out how to rearrange this ugly formula to get the length of one of the sides of a triangle. I've inputted all of the numbers but am struggling with the rearranging. Could someone please go over it with me step by step?

    9.92 = x2+7.12 + x(14.2cos(47))

    I then minused the 7.1 squared to get this:

    9.92 - 7.12 = x2 + x(14.2cos(47))

    then i totalled the left hand side to get this:

    47.6 = x2 + x(14.2cos(47))

    but it is still really disgusting and I can't figure it out help?
    Assuming you were right to use the cos rule here (I suspect you might have chosen the wrong sides to substitute), you could complete the square. Evaluate cos47.
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Kvothe the arcane)
    Assuming you were right to use the cos rule here (I suspect you might have chosen the wrong sides to substitute), you could complete the square. Evaluate cos47.
    I have got A(47o) , c(7.1cm) and a(9.9cm) and I'm trying to work out b(x)

    I've put cos47 in the calc and come out with 0.6819983601

    The homework is on the cosine rule so I'd be worried if it wasn't
    • Community Assistant
    • Very Important Poster
    Offline

    20
    ReputationRep:
    Community Assistant
    Very Important Poster
    (Original post by Bhansongal98)
    I have got A(47o) , c(7.1cm) and a(9.9cm) and I'm trying to work out b(x)

    I've put cos47 in the calc and come out with 0.6819983601

    The homework is on the cosine rule so I'd be worried if it wasn't
    Any chance you can post a diagram?

    I would have worked out angle B using the sin rule.

    And then said that b^2=a^2+c^2-2ac \cos B

    Now that you have 0.682, you can substitute it into your equation and complete the square (or use quadratic formula) to get an answer for x. You can disregard the negative solution.

    47.6 = x^2 - x(14.2(0.682))
    (x-a)^2=b^2
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Kvothe the arcane)
    Any chance you can post a diagram?

    I would have worked out angle B using the sin rule.

    And then said that b^2=a^2+c^2-2ac \cos B

    Now that you have 0.682, you can substitute it into your equation and complete the square to get an answer for x. You can disregard the negative solution.
    Attached Images
     
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Kvothe the arcane)
    Any chance you can post a diagram?

    I would have worked out angle B using the sin rule.

    And then said that b^2=a^2+c^2-2ac \cos B

    Now that you have 0.682, you can substitute it into your equation and complete the square (or use quadratic formula) to get an answer for x. You can disregard the negative solution.

    47.6 = <i>x</i><sup>2 </sup>- <i>x</i>(14.2(0.682))
    (x-a)^2=b^2
    I got 3.6 to 1dp? does this sound right to you? I completed the square like you said
    • Community Assistant
    • Very Important Poster
    Offline

    20
    ReputationRep:
    Community Assistant
    Very Important Poster
    (Original post by Bhansongal98)
    I got 3.6 to 1dp? does this sound right to you? I completed the square like you said
    I think you had a - error so you got the wrong solution
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Kvothe the arcane)
    I think you had a - error so you got the wrong solution
    :confused::confused::confused::confused:
    • Community Assistant
    • Very Important Poster
    Offline

    20
    ReputationRep:
    Community Assistant
    Very Important Poster
    (Original post by Bhansongal98)
    :confused::confused::confused::confused:
    9.92 - 7.12 = x2 + x(14.2cos(47))
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by Kvothe the arcane)
    9.92 - 7.12 = x2 + x(14.2cos(47))

    Ah cool, I mixed my signs then? I've got to go back to class now but thank you kindly for your help!
    • Community Assistant
    • Very Important Poster
    Offline

    20
    ReputationRep:
    Community Assistant
    Very Important Poster
    (Original post by Bhansongal98)
    Ah cool, I mixed my signs then? I've got to go back to class now but thank you kindly for your help!
    Yup. No worries. I'm glad to be of assistance .
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: March 7, 2016
The home of Results and Clearing

1,854

people online now

1,567,000

students helped last year

University open days

  1. Bournemouth University
    Clearing Open Day Undergraduate
    Wed, 22 Aug '18
  2. University of Buckingham
    Postgraduate Open Evening Postgraduate
    Thu, 23 Aug '18
  3. University of Glasgow
    All Subjects Undergraduate
    Tue, 28 Aug '18
Poll
How are you feeling about GCSE results day?
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.