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  1. #1

    need math help part2

    need to understand this type of problem for the test tomorrow:

    "You buy a car for $24,900. it decreases 10% per year. write an exponential decay model for the value of the car. After how many years will the car be worth half of the purchase price?"

    the formulas we are supposed to work with:
    A=P(1 + %/n) ^nt
    and
    Pe^rt

    thx in advanced

  2. #2

    you're fucked.

  3. #3

    Quote Originally Posted by aurik
    you're fucked.

  4. #4
    Melee Summoner
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    24,900 / 2 = 12,450
    So set it as 12,450 = 24,900 * e^ (.10)(t) and solve for t to get the answer.

  5. #5

    Quote Originally Posted by aireeku
    24,900 / 2 = 12,450
    So set it as 12,450 = 24,900 * e^ (.10)(t) and solve for t to get the answer.
    ty
    so do i do it like this?

    1/2 = 1.105t
    .452 = t

    doesnt look right
    or am i supposed to do LN?

  6. #6
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    Has he taught you what 'e' really is yet?

    I'm guessing n is the rate, so if it was decreasing quarterly, we would use 4 for n. But because it is infinately decreasing, we just treat (1+r/n)^nt as e^rt (which you can prove using limits later on).

    Since you want to find when the car will be worth 12,450, you set that as A.

    12,450=Pe^rt
    r=.1, or 10%
    P= 25,900
    t= the amount of time it takes for the price to get to 12,450, which is what you want, so you solve for that.
    .5 = e^.1t
    ln |.5| = .1t
    Divide by .1 and you have your answer.
    You need to do ln to cancel out the e

    For my test on that shit, we had to prove e lol

    Edit: lol, forgot to add P, thought it didn't look right

  7. #7

    Quote Originally Posted by Demosthenes11
    Has he taught you what 'e' really is yet?

    I'm guessing n is the rate, so if it was decreasing quarterly, we would use 4 for n. But because it is infinately decreasing, we just treat (1+r/n)^nt as e^rt (which you can prove using limits later on).

    Since you want to find when the car will be worth 12,450, you set that as A.

    12,450=Pe^rt
    r=.1, or 10%
    P= 25,900
    t= the amount of time it takes for the price to get to 12,450, which is what you want, so you solve for that.
    .5 = e^.1t
    ln |.5| = .1t
    Divide by .1 and you have your answer.
    You need to do ln to cancel out the e

    For my test on that shit, we had to prove e lol

    Edit: lol, forgot to add P, thought it didn't look right
    i was absent when he taught the ln and e stuff

    thx. hope i get at least a b- :DDD
    edit: wtf
    ln(.5) = .1t
    -6.931 = t

    so ~7 years?
    ?_?

  8. #8

    Quote Originally Posted by untouchable
    Quote Originally Posted by Demosthenes11
    Has he taught you what 'e' really is yet?

    I'm guessing n is the rate, so if it was decreasing quarterly, we would use 4 for n. But because it is infinately decreasing, we just treat (1+r/n)^nt as e^rt (which you can prove using limits later on).

    Since you want to find when the car will be worth 12,450, you set that as A.

    12,450=Pe^rt
    r=.1, or 10%
    P= 25,900
    t= the amount of time it takes for the price to get to 12,450, which is what you want, so you solve for that.
    .5 = e^.1t
    ln |.5| = .1t
    Divide by .1 and you have your answer.
    You need to do ln to cancel out the e

    For my test on that shit, we had to prove e lol

    Edit: lol, forgot to add P, thought it didn't look right
    i was absent when he taught the ln and e stuff

    thx. hope i get at least a b- :DDD
    edit: wtf
    ln(.5) = .1t
    -6.931 = t

    so ~7 years?
    ?_?
    Yes that is Correct, Exponential decay problems are fun

  9. #9

    Where the hell were you guys when I was failing math! ; ;

  10. #10
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    Actually any car you buy is worth %50 less the minute you drive it off the lot.


  11. #11

    Why does that equation just seem to complicate the problem for me? I dunno...I've always had my own way of figuring math problems out, usually ending up being figuring it out backwards. I'm wierd like that.

  12. #12
    Relic Shield
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    rule of 70 (also known as the rule of 72)

    divide 70 by the % and that's the number of years (or whatever time units) it will take to double (or half if it's going down)

  13. #13
    Relic Weapons
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    Quote Originally Posted by Rafbacca
    Actually any car you buy is worth %50 less the minute you drive it off the lot.

    Sad but true. Unless its a Volvo XC90 or a Mercedes E class.... the answer is 10 minutes.

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