Turn on thread page Beta
    • Political Ambassador
    • Thread Starter
    Offline

    20
    ReputationRep:
    Political Ambassador
    i was doing some log questions and tried to complete the square of 6x^2 - 28x +16

    which gave me x=3 x=1.65 (must have done something super wrong)

    and then the quad formula gave me x=4 x=2/3 which is the right answer from markscheme.


    this is before i realised i could simplify it and simply factorise, but it made me think do they give different results or did i just do it incorrectly
    Offline

    11
    ReputationRep:
    The quadratic formula comes from completing the square on the quadratic  ax^2 + bx + c. They both give the same answer, because a quadratic always has the same set of solutions.
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by ihatePE)
    i was doing some log questions and tried to complete the square of 6x^2 - 28x +16

    which gave me x=3 x=1.65 (must have done something super wrong)

    and then the quad formula gave me x=4 x=2/3 which is the right answer from markscheme.


    this is before i realised i could simplify it and simply factorise, but it made me think do they give different results or did i just do it incorrectly
    You just completed the square incorrectly.

    https://www.thestudentroom.co.uk/sho....php?t=4246394
    Offline

    11
    ReputationRep:
    The quadratic formula is just a generalised for of completing the square

    x^2 +(b/a)x +(c/a) = 0
    (x+(b/2a))^2 +(4ca-b^2)/(4a^2)=0
    x+(b/2a) = +-sqrt(b^2-4ac)/2a
    x=(b/2a) +-sqrt(b^2-4ac)/2a
    • Political Ambassador
    Offline

    3
    ReputationRep:
    Political Ambassador
    I second what trythis has said above.

    In fact it is very simple to derive the quadratic formula from the general form of a quadratic equation by completing the square. See attachment (apologies for my messy handwriting)
    Attached Images
     
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 28, 2017
Poll
Do you think parents should charge rent?
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.