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Thread: HNC maths help

  1. #1
    CEE
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    Post HNC maths help

    Hello everyone
    Please can any one help me with the following maths problems,. am stuck

    A machine is designed to have 8 speeds ranging from 200 rev/min to 2000rev/min.If the speeds form a geometric progression determine their values.each correct to the nearest whole number
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  2. #2
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    Re: HNC maths help

    Are you stuck because you do not know what a "geometric progression" is?

    A geometric progression is a sequence of numbers of the form $ar^n$ where a and r are fixed numbers. going through 8 speeds, with the first at n= 0, we must go to n= 7. To go from 200 rev/min to 2000/rev/min we must have $a= 200$ and $ar^7= 2000$. From the first, a= 200. So the second equation becomes $200r^7= 2000$ or $r^7= 10$.
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    Re: HNC maths help

    Also, the nth term in a geometric series = a*r^(n-1)
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    CEE
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    Re: HNC maths help

    Quote Originally Posted by HallsofIvy View Post
    Are you stuck because you do not know what a "geometric progression" is?

    A geometric progression is a sequence of numbers of the form $ar^n$ where a and r are fixed numbers. going through 8 speeds, with the first at n= 0, we must go to n= 7. To go from 200 rev/min to 2000/rev/min we must have $a= 200$ and $ar^7= 2000$. From the first, a= 200. So the second equation becomes $200r^7= 2000$ or $r^7= 10$.
    Thank you very munch for the response, I do understand what is a "geometric progression" but I can't seem to work out this sequence using the formula and given the first term ,last term and common difference r^7 =10=22
    Using the formula Xn=ar^n-1. For example second term should be : X2=200x22^2-1= 4427, which is way out of the range 200-2000 given
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    CEE
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    Re: HNC maths help

    Thank you, i have the formula but I can't work out the geometric sequence required given the first term and last term
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    Re: HNC maths help

    Quote Originally Posted by CEE View Post
    Thank you very munch for the response, I do understand what is a "geometric progression" but I can't seem to work out this sequence using the formula and given the first term ,last term and common difference r^7 =10=22
    Uh, 10 is not equal to 22! Did you intend to say that "since $r^7= 10$, r= 22? But that is certainly not true!

    Using the formula Xn=ar^n-1. For example second term should be : X2=200x22^2-1= 4427, which is way out of the range 200-2000 given[/QUOTE]
    Where did you get "22" from? I said before that $r^7= 10$ so that $r= 10^{1/7}= 1.3895$. then "$ar^{2-1}= 200(1.3895)= 277.90$".
    Last edited by HallsofIvy; Jan 21st 2018 at 11:11 AM.
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    Re: HNC maths help

    Quote Originally Posted by CEE View Post
    ...and common difference r^7 =10=22
    Look buddy, you need classroom help.
    A geometric sequence has a common RATIO, not a common DIFFERENCE.
    Common difference applies to arithmetic sequences.
    RULE:
    x^p = y
    x = y^(1/p)
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  8. #8
    CEE
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    Re: HNC maths help

    Thank you , it works . i think you are right
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  9. #9
    CEE
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    Re: HNC maths help

    Thank you very much, it works . I need to recap my knowledge in Rules of indices . i think is what let me down
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