The Official Math/Physics/Whatever Homework questions thread - Page 103

I'm trying to help my son with high school algebra but need some refreshers of my own.

Math degree + time, where time > 20 years = lol wtf omg

I found this site: http://www.purplemath.com/modules/index.htm

and it is good. Are there any others you recommend?

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02-23-2012 , 03:50 PM

#2552

Wyman

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Join Date: Mar 2007 Posts: 13,011

There are some (free) videos that people seem to like at khanacademy.com

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02-23-2012 , 04:01 PM

#2553

Cueballmania

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You should be able to just skim his book and pick it up pretty quickly IMO.

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02-23-2012 , 07:45 PM

#2554

The Undecider

centurion

Join Date: Nov 2010 Posts: 103

Quote:

Originally Posted by Max Raker

Is there some sort of rounding issue? You got pretty closet to 0.

I don't believe so, I'll ask my professor again tomorrow because I have no idea what i did wrong.

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02-24-2012 , 07:25 AM

#2555

riske

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Join Date: Jan 2005 Posts: 3,493

I want to show that for the number of integer compositions of n with largest part at most m, p(n,m) the following holds

Using the generating function I have managed to deduce that this should be equal to

but I cannot figure out how to continue from here, please help.

Last edited by riske; 02-24-2012 at 07:32 AM.

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02-24-2012 , 09:02 AM

#2556

muttiah

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I might try to write the product as a sum. What are the coefficients of the polynomial when you expand out the (1-x^i) terms.

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02-24-2012 , 03:11 PM

#2557

The Undecider

centurion

Join Date: Nov 2010 Posts: 103

Could someone please tell me if I did this differential equation correct:

y''''''+64y=0 <------- (6th derivative of y + 64y = 0)

r^6 + 64r = 0

r(r^5 + 64) = 0

r^5 = -64

r^2 = cubed root(-64) = -4

r = sqrt(-4) = Sqrt(4)i = 2i

So, r = 0 and 2i

A + Bcos(2t) + Csin(2t) = 0

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02-24-2012 , 03:25 PM

#2558

Cueballmania

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No.

You're substituting y=e^r. Your first line should be
r^6y+64y=0 -> r^6+64=0

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02-24-2012 , 04:44 PM

#2559

The Undecider

centurion

Join Date: Nov 2010 Posts: 103

Quote:

Originally Posted by Cueballmania

No.

You're substituting y=e^r. Your first line should be
r^6y+64y=0 -> r^6+64=0

wow, i feel stupid, I have no idea why I solved it the way I did. (I usually do it the way you told me.)

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02-24-2012 , 07:19 PM

#2560

riske

veteran

Join Date: Jan 2005 Posts: 3,493

Quote:

Originally Posted by muttiah

I might try to write the product as a sum. What are the coefficients of the polynomial when you expand out the (1-x^i) terms.

Tried doing it but it doesn't seem to work... wonder if I'm missing something obvious.

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02-25-2012 , 12:54 AM

#2561

ShipItYo

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Join Date: Nov 2008 Posts: 2,041

ok im doing this lab for my exercise physiology class... this is basic multiplication / division... but i feel like im doing order of operations wrong here because later on in the lab my answers dont match up... can someone do this equation and tell me what you get so i can compare to my answer...? if you can show work i'd appreciate it as well..

(this is exactly how the equation is written by my professor so..)

VO2(L/min) = (7.73/100) * (((100-16.75-3.56) * .265) - 16.75)

then

VO2(L/min) = (39.95/100) * (((100-17.03-3.74) * .265) - 17.03

much appreciated

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02-25-2012 , 12:58 AM

#2562

masopas

enthusiast

Join Date: Dec 2011 Posts: 53

@dealace1- educator.com good but not all free.

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02-25-2012 , 01:04 AM

#2563

Cueballmania

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.3376 for the first one.

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02-25-2012 , 01:22 AM

#2564

ShipItYo

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Join Date: Nov 2008 Posts: 2,041

ok thanks i got that to just making sure im not messing that up somehow

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02-25-2012 , 05:37 AM

#2565

slipstream

journeyman

Join Date: Aug 2007 Posts: 272

Quote:

Originally Posted by riske

I want to show that for the number of integer compositions of n with largest part at most m, p(n,m) the following holds

Using the generating function I have managed to deduce that this should be equal to

but I cannot figure out how to continue from here, please help.

I (straightforwardly) got the same generating function as you, then spent a while thinking about it before deciding to finally look up the definition of "composition." Your generating function is right if p(n,m) is the number of *partitions* with pieces of size <= m. But in a composition, the arrangement of the terms matter (look it up on Wikipedia).

I've basically figured out the problem now, which I got by working out a recurrence relation for p(n,m); let me know if you're still stuck with the new and improved definition.

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02-25-2012 , 06:58 AM

#2566

riske

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Join Date: Jan 2005 Posts: 3,493

Quote:

Originally Posted by slipstream

Thanks for spotting the difference, would'd been stuck forever otherwise. Will see if I can solve it now!

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02-25-2012 , 11:27 AM

#2567

Wyman

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Quote:

Originally Posted by ShipItYo

ok thanks i got that to just making sure im not messing that up somehow

google (or wolframalpha.com) will both do this for you. Copy and paste your expression into google search and it will do the computation

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02-28-2012 , 06:58 PM

#2568

adacan

old hand

Join Date: Nov 2008 Posts: 1,276

Find the area of the parallelogram with vertices O (0,0,0) Q (2,4,8) and P (1,4,10)

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02-28-2012 , 08:46 PM

#2569

TensRUs

I need a hug

Join Date: Jan 2011 Posts: 5,578

If I'm having a hard time understanding Physics (Mechanics), how screwed am I as a Civil Engineering major?

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02-28-2012 , 10:28 PM

#2570

checkm8

old hand

Join Date: Dec 2007 Posts: 1,495

How screwed you will be is a function of how hard you try.

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02-29-2012 , 02:30 AM

#2571

Banzai-

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Join Date: Jan 2007 Posts: 2,714

Adacan: Use the identity (x-y)(x+y) = x^2-y^2 repeatedly on Heron's Formula for the area of a triangle with sidelengths a, b, c:

A^2 = s(s-a)(s-b)(s-c),
for s = (a+b+c)/2

to see that

A^2 = [(2ab)^2 - (a^2+b^2-c^2)^2]/16

(id actually do this calculation, especially if its a homework problem).

Then find a, b, and c and plug them into the above to find the area of the triangle with the given endpoints. Note that in this case a, b and c are irrational which is why we modified the formula for A. Finally, double A to find the area of the Parellelogram.

Last edited by Banzai-; 02-29-2012 at 02:42 AM. Reason: Edit: Should be right now

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02-29-2012 , 03:09 AM

#2572

masque de Z

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Quote:

Originally Posted by adacan

Find the area of the parallelogram with vertices O (0,0,0) Q (2,4,8) and P (1,4,10)

Or you can use the vector product (its magnitude since the vector product is itself a vector) of the 2 vectors that are defined by the origin and the 2 vertices.

See here in the geometric meaning section

http://en.wikipedia.org/wiki/Vector_cross_product

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02-29-2012 , 03:18 AM

#2573

slipstream

journeyman

Join Date: Aug 2007 Posts: 272

Quote:

Originally Posted by masque de Z

+1. This is definitely the "right" way to do this problem, and abuses the fact that |a x b| = |a| |b| sin(theta). But Banzai's way is probably correct if you haven't studied vectors yet.