Turn on thread page Beta
    • Thread Starter
    Offline

    8
    ReputationRep:
    Hi, please could someone help me with the last part of this question. I don't quite understand from the mark scheme how you get to the answer. I'll just put the other parts of the question in before for context.
    In the previous parts of the question you have to complete the square on 2x^2+6x+5 which I unmderstand and then write down the minimum value for the second part, which would be 1/2.

    And then it says if point A has co-ordinates (-3,5) and point B has the co-ordinates (x,3x+9) show that AB^2 = 5(2x^2 +6x +5), which I understand how to do but then the last part says
    Using your answer from part a).ii (which was 1/2) find the minimum value of the length AB as x varies giving your answer in the form 1/2(square root of n) where n is an integer.
    Sorry if that is really confusing . If it helps the links to the past paper and mark scheme are here.
    http://filestore.aqa.org.uk/subjects...1-QP-JUN13.PDF - question paper
    http://filestore.aqa.org.uk/subjects...W-MS-JUN13.PDF - mark scheme
    Thanks
    Offline

    4
    ReputationRep:
    (Original post by ruby_zara)
    Hi, please could someone help me with the last part of this question. I don't quite understand from the mark scheme how you get to the answer. I'll just put the other parts of the question in before for context.
    In the previous parts of the question you have to complete the square on 2x^2+6x+5 which I unmderstand and then write down the minimum value for the second part, which would be 1/2.

    And then it says if point A has co-ordinates (-3,5) and point B has the co-ordinates (x,3x+9) show that AB^2 = 5(2x^2 +6x +5), which I understand how to do but then the last part says
    Using your answer from part a).ii (which was 1/2) find the minimum value of the length AB as x varies giving your answer in the form 1/2(square root of n) where n is an integer.
    Sorry if that is really confusing . If it helps the links to the past paper and mark scheme are here.
    http://filestore.aqa.org.uk/subjects...1-QP-JUN13.PDF - question paper
    http://filestore.aqa.org.uk/subjects...W-MS-JUN13.PDF - mark scheme
    Thanks
    The minimum value for 2x2 + 6x + 5 is 1/2, hence the minimum point for AB2 is equal to 5 times the minimum value of 2x2 + 6x + 5 (which is a 1/2), hence 5 x 1/2 = 5/2.

    But they wanted the minimum value of AB, not (AB)2, so square root this value. √5/2

    √5/2 = √5/√2 Now rationalise the denominator. √5 x √2 / √2 x √2 = √10/2 = 1/2√10.
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by tajtsracc)
    The minimum value for 2x2 + 6x + 5 is 1/2, hence the minimum point for AB2 is equal to 5 times the minimum value of 2x2 + 6x + 5 (which is a 1/2), hence 5 x 1/2 = 5/2.

    But they wanted the minimum value of AB, not (AB)2, so square root this value. √5/2

    √5/2 = √5/√2 Now rationalise the denominator. √5 x √2 / √2 x √2 = √10/2 = 1/2√10.
    Thank you so much
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 21, 2017
Poll
Favourite type of bread
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.