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#2 Help Me ! » Differential equation, please help » 2015-04-23 05:52:51

Replies: 2

Hi there,

How do I solve this equation where x is a function of time (t) and a, b and c are constants greater than zero

Its easy if it was not for the "-t" part in the denominator, but now I'm stuck... I'm interested in the solution where t i between 0 and b

Thanks in advance

#3 Re: Help Me ! » A number theory problem » 2014-05-11 04:42:12

bob bundy wrote:

hi engrymbiff

has roots w and z

What variable(s) are we talking about here?

eg.  in

the roots for x are 2 and 3


Sorry for my sloppy formulation, I've reformulated the problem (see above).

#4 Help Me ! » A number theory problem » 2014-05-09 06:38:48

Replies: 18

Assume that x and y are whole numbers and prove that

is satisfied by w and z (where w and z also is whole numbers) if and only if the product of x and y is even.

#5 Re: Help Me ! » Fit y = a + (b1*x1+b2*x2+...+bn*xn) + (c1*x1^2+c2*x2^2+...+cn*xn^2) » 2014-02-13 05:53:21

bobbym wrote:

If you post the data I can fit it.

Do you know any Matlab functions for multivariate analysis? Actually, I do not know for certain that every variable from x1 to xn is equally important in explaining y

I do not use matlab so offhand I do not. Your best bet is the help or a forum devoted to matlab.

It is highly unlikely that each x^n will contribute to y in a meaningful way. Many will just be noise. After x^15 that usually is true.

The standard polynomials do not form an orthogonal set and therefore produce much roundoff error.

OK, I'll be back at the office tomorrow and will send you the data then.

Ps. It's xn^2, not x^n (the polynomial is of order two)

#6 Re: Help Me ! » Fit y = a + (b1*x1+b2*x2+...+bn*xn) + (c1*x1^2+c2*x2^2+...+cn*xn^2) » 2014-02-13 04:59:45

But how? Please don't be ironic and it is not a homework!!

Do you know any Matlab functions for multivariate analysis? Actually, I do not know for certain that every variable from x1 to xn is equally important in explaining y

Maybe by polyfitn and polyvaln in Matlab smile

#7 Help Me ! » Fit y = a + (b1*x1+b2*x2+...+bn*xn) + (c1*x1^2+c2*x2^2+...+cn*xn^2) » 2014-02-13 04:07:15

Replies: 5


I have a big set of data and I belive my data can be explained/estimated by the following formula
y = a + (b1*x1+b2*x2+...+bn*xn) + (c1*x1^2+c2*x2^2+...+cn*xn^2)

I want to estimate all coeficcients (a, b1, b2, ..., bn, c1, c2, ..., cn), which in my particular case is 47 coefficients (because n=23 => 1+23*2=47). How do I proceed for doing this in Matlab or Excel?

Please help


Ps. I have 744 samples of y, x1, x2, ..., xn. Actually, I'm trying to explain how my 23 parameters affects y out of a data set containing 1 month of hourly registrations of y, x1, x2, ..., xn, which makes 31x24 = 744 samples).

#10 Re: Help Me ! » maximize (a-b)(a-c)(a-d)(b-c)(b-d)(c-d) s.t. -1 <= a, b, c, d <= 1 » 2012-10-29 05:51:48


L(a,b,c,d,u1,u2,u3,u4,u5,u6,u7,u8) = (a-b)(a-c)(a-d)(b-c)(b-d)(c-d) - u1(1-a) - u2(1-b) - u3(1-c) - u4(1-d) - u5(1+a) - u6(1+b) - u7(1+c) - u8(1+d)

and then try to identify which a,b,c,d,u1,u2,u3,u4,u5,u6,u7,u8 that satifies dL/da = dL/db = dL/dc = dL/dd = dL/du1 = ... = 0.

I guess that I've made a misstake when I set up the inequality constraint in L as

dL/du1 = a-1 = 0 => a = 1
dL/du5 = -a-1 = 0 => a = -1

so dL/du1 and dL/du5 cannot be equal to zero at the same time.

#11 Help Me ! » maximize (a-b)(a-c)(a-d)(b-c)(b-d)(c-d) s.t. -1 <= a, b, c, d <= 1 » 2012-10-28 02:43:12

Replies: 7


Could anybody help me with this problem?

maximize (a-b)(a-c)(a-d)(b-c)(b-d)(c-d) s.t. -1 <= a, b, c, d <= 1

I've tried using Lagrange multipliers but without any luck.

#12 Re: Help Me ! » Show that (413)^(1/3)>6+3^(1/3) » 2012-10-14 05:15:30

Hm... I really cannot solve it, I have a vague memory of how I solved it last time but now I don't have a good clue of how...

#14 Help Me ! » Find the force that empties this bowl containing a liquid... » 2012-08-15 04:42:18

Replies: 2

Here is a picture of the setup

The vessel is attached to a fixed axis (A-B) and has an inner radius of r and outer of R. The mass densities is p for the vessel and p1 for the liquid. Find the force F (at point C in the picture) that holds the vessel at rest with by a horizontal degree b (once again, see the picture).

M = F x L
Vsphere = 4*pi*r^3/3 = > Vvessel = 2*pi*(R^3-r^3)/3
Vliquid = pi*h^2*(3r-h)/3 (where h is the height of the liquid)
h = r(1-sin(b))

First solution wins!

#15 Help Me ! » Two parallel whires, separated by a distance D, ... find the force... » 2012-05-29 06:42:31

Replies: 4

Two parallel whires, separated by a distance D (midpoint distance), are charged with a current density L (each whire has a radius R). If the whires have the potential +/- X Volts, what is the force per unit length between them?

#16 Help Me ! » Find all real sol. to the following system of eq. » 2011-12-12 06:46:40

Replies: 1

Find all real sol. to the following:

How to do this?

#18 Help Me ! » Physics problem » 2011-05-30 04:52:16

Replies: 13

A particle with mass m lies on a circular plate at the distance R from the center point. At time t = 0 the plate begins rotating around a fixed vertical axis through the center. The angular velocity is given by dΘ / dt = αt, where α is a constant. Static friction rate between the plate and the particle is μ. At what time t = T does the particle remains at rest relative to the plate?

#19 Help Me ! » Diff eq. » 2011-05-17 02:23:33

Replies: 1

du/dx=a*du/dy+b*u, where a and b is constants. Initial cond. is given by u(x,0)=c*exp(2x)+d*exp(-3x). Find u(x,y).

Please help

#20 Help Me ! » Conserv. vector field » 2011-02-18 08:24:46

Replies: 1

F is a vector field defined as

F = (2x/(x^2+y^2)-1/x-1/y, 2y/(x^2+y^2)+beta*x/y^2)

where x, y > 0.

For which beta is F conservative? And determine a potential to F with those beta's.

Im stuck// Carl

#21 Help Me ! » integral-problem, spher. and cyl. coord. » 2011-02-15 06:52:46

Replies: 0


Let D be a region within a sphere with radius a and also bounded within the cone z = - (x^2+y^2)^0.5.

1. Describe D with inequalities in spherical as well as cylindrical coordinates.
2. With one of the two in 1., calculate the volume integral of z^2 over the region D.
3. If D where a the volume which you get when you rotate the triangle with corners in the x,z-plane at (0,0), (0,1) and (1,1) around the z-axis and its dencity is constant. At which coordinate would its center of mass be?

#23 Re: Help Me ! » 2 problems: limit and inverse » 2010-09-23 17:16:13

George,y: Sorry but thats not the inverse function

#24 Help Me ! » 2 problems: limit and inverse » 2010-09-22 05:27:34

Replies: 4

I have 2 problems.


What is the limit of

goes to
? (
is a constant)



have an inverse or not? If so, what is it?

/ Thanks and good luck

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