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

    2
    ReputationRep:
    So my weakness, rearranging stuff comes back to haunt me again...
    Q2 is : Find the value of n for which U_n has the given value:
    U_n= (-1)^n \frac{n}{n+4}, U_n= \frac{7}{9}

    So from that, I have

    (-1)^n \frac{n}{n+4}= \frac{7}{9}

    and don't know where to go from there. I'm guessing it involves multiplying out the n+4 but don't really know
    Help please, oh lovely mathemeticians? :o: ty x
    Offline

    2
    ReputationRep:
    I've never seen C1 questions written in Latex before but I'm suitably impressed.

    Notice you want to rewrite 7/9 in a form where the denominator is 4 greater than the numerator, i.e. write 7/9=14/18 which should make the answer clear.
    Offline

    2
    ReputationRep:
    That's a nice method from Gaz there, use that ^^^

    Alternatively, you could do some basic case analysis. Firstly make the assumption that the value of n you're trying to find is even. See if that assumption leads to a solution, but remember you assumed n is even so the only valid solutions to this equation will be even. Then do exactly the same thing except assume that n is odd.

    The idea is that by doing this you can get rid of the (-1)^n. If n is off then it's always equal to -1 whereas if n is even it's always equal to 1. In either case you should have a relatively easy equation to solve.

    But like I say, Gaz's method is much nicer. Use that
    • Thread Starter
    Offline

    2
    ReputationRep:
    Thanks for your help, but I'm afraid I don't get it, the dummy I am

    So now I've got \frac {(-1)^n n}{\frac{14}{18}} = n+4

    I doubt that's right but if it is, I'm stuck again haha. Sorry :P
    Offline

    15
    ReputationRep:
    I think he meant that

    \frac{14}{18} = \frac{n(-1)^n}{n+4}

    You can just read off n from this. It's a clever trick but I think JohnnySPal's method is better for your knowledge.

    *Edit*

    If you're still not following then:

    \frac{14}{14 + 4} = \frac{n(-1)^n}{n+4}
    • Thread Starter
    Offline

    2
    ReputationRep:
    Ooo.. ty :]
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Brussels sprouts
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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.