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Author Topic: Math wizards look here.  (Read 14436 times)

Neko Desu

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Re: Math wizards look here.
« Reply #120 on: January 05, 2012, 07:21:01 pm »

'Sup?

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Dagnarok

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Re: Math wizards look here.
« Reply #121 on: April 19, 2012, 01:45:32 pm »

Pardon the bump, but... I need help.  For seriousness and my own embarrassment. >_>

: Minor background crap • show
I've said this in a few places on the forums (and Facebook, if anyone cares about that), but I'm going into calculus very shortly (I start on the 23rd), along with Bio 30.  Now, Bio 30 won't be so bad because I just finished bio 20 a little over a week ago; I haven't done any serious mathematics since I finished my Math 30 course back in 2005.  Needless to say, the years have taken their toll on my mathematical and problem solving abilities related to said subject.

Before my calculus course starts, we're doing a Math 30 review test the first day; needless to say, I don't care about getting above 80 on it.  I do care about doing at least decently (say, 65) on said test (not to mention Math 30 knowledge in general is gonna vastly help  towards understanding Calculus).  At the rate I'm going with my Math 30 review, Calculus is gonna kick my ass. >_>  Hell, I'll be lucky to PASS THE DAMN REVIEW TEST!  At least in Calculus class I've have help; with my Math 30 review I'm up to my own devices, which, as I've repeated a billion times already, isn't going very well.

So... expect me to be flooding this topic with math questions that I cannot do on my own.  I've lost enough sleep over this, and, quite frankly, I'm just plain sick of staring at both my Math 30 book and my Math 20 and Math 30 study guides.


Here's what I'm currently stuck on:  It's a Logarithm question.

4^X+1 = 5^X-2 (Find the value of X to the nearest hundredth)


I could solve the question just fine if X was only on one side of the equation; but, since there's an X on both sides, I'm completely lost.  I've spent the last hour trying to solve questions like this, and I'm sick of staring at them.
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Nono

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Re: Math wizards look here.
« Reply #122 on: April 19, 2012, 02:35:09 pm »

I started rewriting it and then had a major brain-fart... I'll at least post the rewritten function, perhaps it'll help. Might edit with more if my brain stops being stupid, but I gotta go somewhere right now.
Also, I know the answer. Just not how to get to it :P

I'll assume you mean 4^(X+1) = 5^(X-2).

4^(X+1) = 5^(X-2)
4^(X+1) = 5^X / 5^2
4^(X+1) = 5^X / 25
25 * 4^(X+1) = 5^X

X ≈ 20.64
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Ellipsis

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Re: Math wizards look here.
« Reply #123 on: April 19, 2012, 02:37:45 pm »

Pardon the bump, but... I need help.  For seriousness and my own embarrassment. >_>

: Minor background crap • show
I've said this in a few places on the forums (and Facebook, if anyone cares about that), but I'm going into calculus very shortly (I start on the 23rd), along with Bio 30.  Now, Bio 30 won't be so bad because I just finished bio 20 a little over a week ago; I haven't done any serious mathematics since I finished my Math 30 course back in 2005.  Needless to say, the years have taken their toll on my mathematical and problem solving abilities related to said subject.

Before my calculus course starts, we're doing a Math 30 review test the first day; needless to say, I don't care about getting above 80 on it.  I do care about doing at least decently (say, 65) on said test (not to mention Math 30 knowledge in general is gonna vastly help  towards understanding Calculus).  At the rate I'm going with my Math 30 review, Calculus is gonna kick my ass. >_>  Hell, I'll be lucky to PASS THE DAMN REVIEW TEST!  At least in Calculus class I've have help; with my Math 30 review I'm up to my own devices, which, as I've repeated a billion times already, isn't going very well.

So... expect me to be flooding this topic with math questions that I cannot do on my own.  I've lost enough sleep over this, and, quite frankly, I'm just plain sick of staring at both my Math 30 book and my Math 20 and Math 30 study guides.


Here's what I'm currently stuck on:  It's a Logarithm question.

4^X+1 = 5^X-2 (Find the value of X to the nearest hundredth)


I could solve the question just fine if X was only on one side of the equation; but, since there's an X on both sides, I'm completely lost.  I've spent the last hour trying to solve questions like this, and I'm sick of staring at them.

I'm fairly sure it's not possible. The solution is likely transcendental, that is to say you can't express it because it's irrational.
Or that is to say, not possible in a nice way.
Newton–Raphson is a possibility.
There's no easy way to proceed here.
I'm not sure what level you're doing the maths at but it's a strange question to get.
The solution is about 1.473 if your interested.

Bhagh, ninja'd.

Ok. Take two with the new question:
4^(X+1) = 5^(X-2)
4^(X+1) = 5^(X-2)
4^(X+1) = 5^X/5^2
4^(X+1) = 5^X/25
25*4^(X+1) = 5^X
100*4^x=5^x
log 100 + xlog4 = xlog5
2=xlog5-xlog4
2/(log5-log4)=x

Hope this helps.
« Last Edit: April 19, 2012, 02:49:44 pm by ... »
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Venser

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Re: Math wizards look here.
« Reply #124 on: April 19, 2012, 03:04:01 pm »

You don't double-post, Ellipsis. First off, edits of the last post in the thread cause the forum to reregister the thread as "unread". Second, you should instead just copy the text of the first post, delete it, and then repost it with your edit if it really merits a bump.
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Kudo.

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Re: Math wizards look here.
« Reply #125 on: April 19, 2012, 03:14:01 pm »

I was going to pick up from where Nono left off, but Ellipsis fixed his own work.

WHATEVER. HERE'S ANOTHER WAY TO LOOK AT IT:

from: 25 * 4^(X+1) = 5^X
100 * 4^X= 5^X
100= (5^X)/(4^X)
100= (5/4)^X
log(base 5/4)(100)= X
log(100)/log(5/4)= X
log(100)/(log(5)-log(4))= X

...basically exactly what Ellipsis gets, and fits Nono's answer.
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Dagnarok

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Re: Math wizards look here.
« Reply #126 on: April 19, 2012, 04:14:19 pm »

I'm not sure what level you're doing the maths at but it's a strange question to get.

Senior High School, or Grade 12. :P  Yeah, I did Grade 12 Math in my Sophomore year; I wanted it out of my hair ASAP.

Yep; Nono had the right answer, and I was able to answer a few similar questions. ^_^  Now I'm stuck on one of the example questions. XD

For the example question, you have to do the following:  Find X...

-as an exact value
-to three decimal places

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2(3)^(X-2) = 7^X

2(3^X/3^2) = 7^X
2(3^X/9) = 7^X
2(3^X) = 9(7^X)
(3^X)/(7^X) = 9/2

and at this point I'm not sure what to do.  I think I've missed a step, but....


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Ellipsis

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Re: Math wizards look here.
« Reply #127 on: April 19, 2012, 04:23:39 pm »

2(3)^(X-2) = 7^X
2*(1/9)*3^x=7^x
log((2/9)*3^x)=log(7^x)
log(2/9) + xlog3=xlog7
log(2/9)=xlog7 - xlog3
log(2/9)/(log7-log3)=x

so x=-1.7751...
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Dagnarok

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Re: Math wizards look here.
« Reply #128 on: April 19, 2012, 04:32:38 pm »

2*(1/9)*3^x=7^x

I'm not sure if there's a difference between putting the 3^X inside or outside the bracket; it made sense to me that it should've been inside the bracket because the base was inside a bracket.
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Ellipsis

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Re: Math wizards look here.
« Reply #129 on: April 19, 2012, 04:39:22 pm »

Sorry I don't understand.
What did I do wrong?
How should it read?
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Venser

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Re: Math wizards look here.
« Reply #130 on: April 19, 2012, 04:40:19 pm »

Just so you know, there exist no actual applications of knowing specific rules of logarithms in higher mathematics. Anything you can do with this, a computer can do better; granted, when you take Calculus they may have you do derivatives and integrals where knowing the rules will aid you in solving the problem, but again, a computer can always do it better. So don't worry yourself overmuch about these: it's not something you need to know for anything but the test, honestly.

Admittedly much of lower-level mathematics is like that, but I figured I'd just tip you off so you don't spend a ton of time trying to comprehend something that has no practical value.
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Dagnarok

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Re: Math wizards look here.
« Reply #131 on: April 19, 2012, 04:43:52 pm »

It's not that I'm saying you're wrong; I'm just wondering why you wrote that part of the work the way you did. :P

(ninja'd)

Truth; I actually have the Calculus book we're gonna be using for the course (I would've gotten it earlier, but it wasn't certain that said book would still be in use).  It's still important that I understand the train of thought regarding it when I go into the course, because we're likely going to be starting on the lower-level mathematics anyway.
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Ellipsis

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Re: Math wizards look here.
« Reply #132 on: April 19, 2012, 05:08:53 pm »

Ah ok.
Well I just write it like that as I'm copying it from my notes on a scrap of paper. I always write a ninth (1/9) and then the power next to it.
I think of 3^(x+1) as 3*3^x so therefore I also think of 3^(x-1) as (1/3)*3^x.
It's just my strange way of thinking of things.
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Neko Desu

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Re: Math wizards look here.
« Reply #133 on: April 30, 2012, 09:33:27 pm »

FAILING ON CALC.

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Dagnarok

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Re: Math wizards look here.
« Reply #134 on: April 30, 2012, 10:47:43 pm »

I'm guessing that's Trig, and... well... I don't start that until June. :P
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