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Math

Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
edited November 2011 in Help / Advice Forum
Me again, with more math problems. I'm on financial formulas, and I'm about to claw my own eyeballs out in frustration.

$45.67 at 3.5% compounded daily interest for 3 years. Now, it should look like this; 45.67(1+3.5/365)^(365*3), right? But I get a nonsensical number; 1577605.40. That's way too big to be right, isn't it? I'm so frustrated. I've never been taught this stuff and now I'm on the verge of failing math class. Trying to find the future value.

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  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    Just to be clear I'm using the right formula.

    A=P(1+(r/m))^rt

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  • SavantSavant Registered User regular
    Er...3.5% = .035. You have it being simply 3.5 outright in your equation, which of course is a HUGE number in terms of compounding interest.

  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    Ah, crap! I'm an idiot. Thanks! I really have to pay attention to what I'm doing. I'll keep the thread open, because I'll inevitably get stuck again.

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  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    Man, fuck this shit. I'm failing this class anyways. Might as well drop it and take it again next semester. I suppose I'll have to live with my 3.5 GPA dropping to 2.5 I only hope I can still get enough financial aid to attend. God damn, mother-fucking, son of a bitch.

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  • faerielaurelynfaerielaurelyn Registered User regular
    There's a really really simple way to do it on a non-programmable scientific calculator, where you just input the numbers and it does the calculations for it. PM me if you want to talk about it more.

    We just got through compounded interest in business math, and I struggled with it as well, but it's not so bad and I'd be happy to try and help you get through it too!

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  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    edited November 2011
    I'll do that, thanks faerielaurelyn. I actually another question now, my homework gives me a half-life question, but I'm not sure it gives me enough info for the answer.

    "If the half-life of (whatever) is 25 years, find the decay constant r."

    Do I just assume it starts from 100 mg?

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  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    I think I got it.

    50=100e^r(35)
    0.5=e^35r

    35r/35=ln0.5/35

    r=ln0.5/35

    r=-0.0198 (Had to round it to 4 decimal places)

    Is this right?

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  • khainkhain Registered User regular
    I believe it should be half-life = ln(2)/decay constant.

  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    I have the half-life, I need to find the decay constant.

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  • SavantSavant Registered User regular
    I'll do that, thanks faerielaurelyn. I actually another question now, my homework gives me a half-life question, but I'm not sure it gives me enough info for the answer.

    "If the half-life of (whatever) is 25 years, find the decay constant r."

    Do I just assume it starts from 100 mg?
    I think I got it.

    50=100e^r(35)
    0.5=e^35r

    35r/35=ln0.5/35

    r=ln0.5/35

    r=-0.0198 (Had to round it to 4 decimal places)

    Is this right?

    You need to be paying closer attention to what you are doing, because you have 35 years in one place and say 25 years in another. Which one is it?

    Also, the decay constant should be positive if it is actually decreasing in value over time. Look over http://en.wikipedia.org/wiki/Exponential_decay or http://en.wikipedia.org/wiki/Half-life to see some examples of the formulas and derivations that seem like they may be relevant on this subject.

  • khainkhain Registered User regular
    You can just multiply both sides by decay constant and 1/half-life and you get decay constant = ln(2)*half-life. Anyway, it will give you the same number, except decay rates are typically given as positive and the corresponding formula would be N(t) = N(0) e^-rt for exponential decay.

  • Gigazombie CybermageGigazombie Cybermage Registered User, __BANNED USERS regular
    edited November 2011
    Thanks for the help. I've got some more questions. Sorry for being annoying, but I'm right on the brink of failing Math and wrecking my GPA.

    Does this look okay?

    600(((1+0.06(1/4))^(4(20))

    Trying to find an investment deposited quarterly, compounded quarterly after twenty years at 6%

    I got $12,736.36 as an answer

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  • ToxTox I kill threads Registered User regular
    Thanks for the help. I've got some more questions. Sorry for being annoying, but I'm right on the brink of failing Math and wrecking my GPA.

    You should look into a tutor. Most colleges have some sort of learning center that can set you up with someone, and it's free of charge. Actually that's not true, it's paid for out of your tuition expenses, so you're already paying for a tutor anyway.

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  • SavantSavant Registered User regular
    Thanks for the help. I've got some more questions. Sorry for being annoying, but I'm right on the brink of failing Math and wrecking my GPA.

    Does this look okay?

    600(((1+0.06(1/4))^(4(20))

    Trying to find an investment deposited quarterly, compounded quarterly after twenty years at 6%

    I got $12,736.36 as an answer

    You're typing something in wrong somewhere when computing it, which you should notice because that number looks like it should be rather too high for 20 years of compounding interest on $600. The future value of 600 after 20 years compounded quarterly is $1974.40. Which is 600*((1.015)^80).

  • ED!ED! Registered User regular
    Exponential growth is always positive; exponential decay is always negative. Having a positive constant doesn't make any sense for a question about half life.
    Thanks for the help. I've got some more questions. Sorry for being annoying, but I'm right on the brink of failing Math and wrecking my GPA.

    Does this look okay?

    600(((1+0.06(1/4))^(4(20))

    Trying to find an investment deposited quarterly, compounded quarterly after twenty years at 6%

    I got $12,736.36 as an answer

    The formula you have quoted assumes an individual has made an initial investment (of 600 bucks), and that investment is earning interest on it (or rather that's what it loos like to me). If you are told that a person is making periodic deposits into an account, that is earning interest at a certain rate, you want to use a different formula. I'd have to dig up old Business Math formula sheets (as I can rarely remember them myself), but what you have quoted isn't it.

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  • y2jake215y2jake215 The No Flex ZoneRegistered User regular
    edited November 2011
    ED! wrote:
    Exponential growth is always positive; exponential decay is always negative. Having a positive constant doesn't make any sense for a question about half life.

    Generally (at least in my graduate radiation courses) the decay constant is given as a positive value, the negative sign is added in the formula
    N(t) = N(0)e^-[(decay constant)*(time)]

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  • ED!ED! Registered User regular
    That is how most teach it, and it helps keep students from getting confused. I've always stressed to students that it is the negative acting on the constant that gives us a decreasing function of time. Realistically they'll get confused either way.

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