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Math Help - Expanding, factorising and transposing questions

  1. #1
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    Expanding, factorising and transposing questions

    1) expand and simplify:

    (x-1/x)2

    2) factorise:
    x^2+6x+8

    3) transpose to make t the subject:
    v=200e^(-Rt/L)

    Last edited by mr fantastic; December 8th 2009 at 02:35 AM. Reason: Changed post title
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  2. #2
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    Quote Originally Posted by clayson View Post
    1) expand and simplify:

    (x-1/x)2

    2) factorise:
    x^2+6x+8

    3) transpose to make t the subject:
    v=200e^(-Rt/L)


    1) \frac{2x-2}{x}

    2) (x+2)(x+4)

    3) log? or ln?
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  3. #3
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    yep base e

    3) log?[/quote]
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  4. #4
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    Quote Originally Posted by clayson View Post
    yep base e

    3) log?
    Got it.

    t=\frac{ln\frac{v}{200}L}{-R}

    :P
    Last edited by BabyMilo; December 7th 2009 at 08:23 AM.
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  5. #5
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    The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

    R=150 ohms
    L=12.5*10^-3 H

    What time does it take for the voltage to reach 85 volts>

    could you go through it step by step if possible and also for the (x-1/x)^2 question.

    Thanks BabyMilo for the quick reply.
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  6. #6
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    Quote Originally Posted by clayson View Post
    The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

    R=150 ohms
    L=12.5*10^-3 H

    What time does it take for the voltage to reach 85 volts>

    could you go through it step by step if possible and also for the (x-1/x)^2 question.

    Thanks BabyMilo for the quick reply.
    for a) which one is it? (x-1/x)^2 or (x-1/x)2?

    they are quite different.!
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  7. #7
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    Quote Originally Posted by clayson View Post
    The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

    R=150 ohms
    L=12.5*10^-3 H

    What time does it take for the voltage to reach 85 volts>

    could you go through it step by step if possible and also for the (x-1/x)^2 question.

    Thanks BabyMilo for the quick reply.
    v = 200e^{\frac{-Rt}{L}}

    \frac{v}{e^{\frac{-Rt}{L}}} = 200

    e^{\frac{-Rt}{L}} = \frac{v}{200}

    \frac{-Rt}{L} = ln(\frac{v}{200})

    -Rt = ln(\frac{v}{200})*L

    t = \frac{ln(\frac{v}{200})*L}{-R}

    sub in the numbers.

    t = \frac{ln(\frac{85}{200})*12.5*10^{-3}}{-150}

    t = 71.3110^−6s
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  8. #8
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    (x-1/x)^2 sorry for the mistake.
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  9. #9
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    Quote Originally Posted by clayson View Post
    (x-1/x)^2 sorry for the mistake.
    <br />
(\frac{x-1}{x})^2

    <br />
\frac{(x-1)^2}{x^2}

    \frac{1}{x^2}-\frac{2}{x}+1
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