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    The price of a computer system can be modelled by the formula:
    P = 100 + 850e^{\frac{-t}{2} }

    When will it be worth less than £200?

    I did:
    100 + 850e^{\frac{-t}{2}} < 200
    850e^{\frac{-t}{2}} < 100
    e^{\frac{-t}{2}} < \frac{2}{17}
    e^{\frac{t}{2}} > \frac{2}{17}
     \frac{t}{2} > In\frac{2}{17}
    t > -4.28

    The answer is 4.28, so I'm doing it right, but why am I getting a negative? Thanks!


    Edit: Is it because I forgot to inverse 2/17 into 17/2, when I removed the - from the t - that gives me the right answer. Sorry guys - but is this right?
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    If it's easy, you would do it.
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    (Original post by Narik)
    The price of a computer system can be modelled by the formula:
    P = 100 + 850e^{\frac{-t}{2} }

    When will it be worth less than £200?

    I did:
    100 + 850e^{\frac{-t}{2}} < 200
    850e^{\frac{-t}{2}} < 100
    e^{\frac{-t}{2}} < \frac{2}{17}
    e^{\frac{t}{2}} > \frac{2}{17}
     \frac{t}{2} > In\frac{2}{17}
    t > -4.28

    The answer is 4.28, so I'm doing it right, but why am I getting a negative? Thanks!


    Edit: Is it because I forgot to inverse 2/17 into 17/2, when I removed the - from the t - that gives me the right answer. Sorry guys - but is this right?
    yeah, you need to use 17/2.

    when you have e^ (-t/2) > 2/17

    it's 1/ e^(t/2) > 2/17

    when you flip it you get e ^ (t/2) = 17/2
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    (Original post by Jingers)
    If it's easy, you would do it.
    The mistake I was making was simple - so finding out where I went wrong wasn't hard.
 
 
 
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