Join TSR now and get all your revision questions answeredSign up now

C3 HW help

    • Thread Starter
    Offline

    2
    ReputationRep:
    Im stuck on this question:

    Show that the composite function gf is (8x^3 - 1) / (1 - 2x^3)

    f(x)= 1 - 2x^3
    g(x)=(3/x) - 4

    I subbed f into g but didnt get it
    any help appreciated


    also, solve gf(x)=0, if i set it = 0 then i cant bring anything across, do i have to factorise and cancel down the bottom maybe?
    Offline

    3
    ReputationRep:
    (Original post by not_lucas1)
    Im stuck on this question:

    Show that the composite function gf is (8x^3 - 1) / (1 - 2x^3)

    f(x)= 1 - 2x^3
    g(x)=(3/x) - 4

    I subbed f into g but didnt get it
    any help appreciated


    also, solve gf(x)=0, if i set it = 0 then i cant bring anything across, do i have to factorise and cancel down the bottom maybe?
    You need to put 1-2x^3 where the x is in g(x) and then put it all over the same denominator
    Offline

    3
    ReputationRep:
    (Original post by not_lucas1)
    Im stuck on this question:

    Show that the composite function gf is (8x^3 - 1) / (1 - 2x^3)

    also, solve gf(x)=0, if i set it = 0 then i cant bring anything across, do i have to factorise and cancel down the bottom maybe?
    0 times anything is 0 so when you multiply up by the denominator you just get the numerator =0
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by tiny hobbit)
    You need to put 1-2x^3 where the x is in g(x) and then put it all over the same denominator
    i did and i didnt get the same answer
    Offline

    1
    ReputationRep:
    (Original post by not_lucas1)
    i did and i didnt get the same answer
    Did you get this? You'd only have to distribute and simplify.

    g\circ f(x)=g(f(x))=g(1-2x^{ 3 })=\frac { 3 }{ 1-2x^{ 3 } } -4=\frac { 3-4(1-2x^{ 3 }) }{ 1-2x^{ 3 } }
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by MartyO)
    Did you get this? You'd only have to distribute and simplify.

    g\circ f(x)=g(f(x))=g(1-2x^{ 3 })=\frac { 3 }{ 1-2x^{ 3 } } -4=\frac { 3-4(1-2x^{ 3 }) }{ 1-2x^{ 3 } }
    No but I think you're correct. I didnt think to put 4/1 and then make it a common denominator (i think that is what you have done anyway)
 
 
 
Poll
Which party will you be voting for in the General Election 2017?
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

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