# Search results

1. ### Matrices with complex entries

Homework Statement Let B be an m×n matrix with complex entries. Then by B* we denote the n×m matrix that is obtained by forming the transpose of B followed by taking the complex conjugate of each entry. For an n × n matrix A with complex entries, prove that if u*Au = 0 for all n × 1 column...
2. ### Fourier sine transform for Wave Equation

But that only makes use of the regular Fourier Transform, and not the Fourier Cosine Transform.
3. ### Fourier sine transform for Wave Equation

[PLAIN]http://img815.imageshack.us/img815/9894/fourier5313.png [Broken] there was a minus sign. oops. In any case, relevant equations: F_{c}(f(x,t)) = \sqrt{\frac{2}{\pi}} \int^{\infty}_{0} cos (\lambda x) f(x,t) dx F_{c}(f''(x,t)) = -\sqrt{\frac{2}{\pi}} f'(0,t) - \lambda^{2} F_{c}(f(x,t))...
4. ### Fourier sine transform for Wave Equation

Homework Statement Find the solution u, via the Fourier sine/cosine transform, given: u_{tt}-c^{2}u_{xx}=0 IC: u(x,0) = u_{t}(x,0)=0 BC: u(x,t) bounded as x\rightarrow \infty , u_{x}(0,t) = g(t) 2. The attempt at a solution Taking the Fourier transform of the PDE, IC and BC...
5. ### Two springs connected by a spring

Well, then I would have m_{2}\ddot{x_{2}}-m_{1}\ddot{x_{1}} = -kq ...which, from what I see, doesn't do a whole lot because I still can't factor the left side to do anything.
6. ### Two springs connected by a spring

Homework Statement Two masses, m1 and m2, are connected to each other by a spring with a spring constant k. The system moves freely on a horizontal frictionless plane. Find the natural frequency of oscillation. Homework Equations F = -kx F = ma The Attempt at a Solution Let m1 be the mass...
7. ### Finite approximation of PDEs

Homework Statement Given u_tt = F(x,t,u,u_x, u_xx), give the finite difference approximation of the pde (ie using u_x = (u(x + dx; t) - u(x - dx; t))/(2dx) etc.) Homework Equations Well, clearly, u_x = (u(x + dx; t) - u(x - dx; t))/(2dx) The Attempt at a Solution I really have no idea how...

bump.
9. ### Introductory PDE (diffusion equation)

Homework Statement u_t = -{{u_{x}}_{x}} u(x,0) = e^{-x^2} Homework Equations The Attempt at a Solution The initial state is a bell curve centred at x=0. The second partial derivative of u at t=0 is {4x^2}{e^{-x^2}}, which is a Gaussian function, which means nothing to me other than its...
10. ### Double integrals in polar coordinates

sorry to do this, but bump.
11. ### Double integrals in polar coordinates

tiny-tim, thanks for those. desperately needed. hallsofivy, I did draw the diagram, and found that it was rather inconveniently symmetrical, which was why I got stumped. going by your suggestion, from θ = 0 to θ = π/2, I would be integrating along the right side of the curve, where the...
12. ### Double integrals in polar coordinates

Homework Statement Find \int{\int_{D}x dA} where D is the region in Q1 between the circles x2+y2=4 and x2+y2=2x using only polar coordinates. The Attempt at a Solution Well, the two circles give me r=2 and r=2 cos \theta, and the integrand is going to be r2cos \theta, but I have no...
13. ### Max/min with partial derivatives

*backtracks* ooooooooooookay. so. when x=0, f_x = -2, but f_x = 0, so x=0 is not a solution. which brings me back to the points (-1,0) and (1,2). looking at my calculations for (-1,0), turns out that it's horrendously wrong. At (-1,0), f_xx = -2(6-2) = -8, D = (-8)(-2) - (0-3+2)^2 = 16 - 1...
14. ### Max/min with partial derivatives

well, from f_y, I can deduce that x=0 or yx^2 - x - 1 = 0. If x=0, from f_x, y= -2. If yx^2 - x - 1 = 0, y = (x+1)/(x^2) Subbing into f_x, -2(x^2 - 1)(2x) - 2((x+1)-x-1)(2x(x+1)/(x^2) - 1) = 0 -2(x^2 - 1)(2x) = 0 x(x^2 - 1) = 0 x = 0 (as above), x = 1 or x = -1 If x = 1, y = 2 If x = -1...
15. ### Maximizing Coffee Profits

It's on the right track, but to finish it off, you should also apply the second derivative test for partial deriviatives.
16. ### Max/min with partial derivatives

Homework Statement Show that f(x,y) = -(x^2 - 1)^2 - (yx^2-x-1)^2 has only two critical points, and both are maxima. The Attempt at a Solution Set partial derivatives (wrt x and y) to zero to find critical pts. f_x = -2(x^2 - 1)(2x) - 2(yx^2 - x - 1)(2xy - 1) = 0 f_y = -2(yx^2 - x -...
17. ### Gradient vector for polar coordinates

ahh. of course. r^2 = x^2 + y^2, and sin theta = y/r, where r is the square root of r^2. I indeed was making it too complicated for myself. thanks everyone.
18. ### Gradient vector for polar coordinates

because I don't know how, but I'll give it a shot. r = x/cos (theta) = y/sin(theta) theta = arccos (r/x) = arcsin (r/y) partial g/partial x = (partial g/partial r)(partial r/partial x) + (partial g/partial theta)(partial theta/partial x) = (-e^(-r))(1/cos(theta)) + (e^(-r)...
19. ### Gradient vector for polar coordinates

I want to convert del g into cartesians so I can find a directional derivative, considering the direction vector is in cartesians. I know how to calculate the directional derivative; I just don't know how to convert del g into cartesians from polars.
20. ### Gradient vector for polar coordinates

Homework Statement Find the gradient vector of: g(r, \theta) = e^{-r} sin \theta Homework Equations The Attempt at a Solution I know how to get gradients for Cartesian - partially derive the equation of the surface wrt each variable. But I have no idea how to do it for...
21. ### Transformation of Coordinate Systems

let's start with the second inequality. xy goes from 1 to 3. this forms one dimension of a rectangle - along the xy-axis. but instead of using an xy-axis, you could use a u-axis or v-axis, if you let u or v equal to xy. Hint hint. now for the first inequality, if y goes from x^2 and 2x^2, is...
22. ### Partial derivatives of implicitly defined functions

Ok, so now I have 3t^2 \frac {\partial s}{\partial y} = -6s \frac{\partial t}{\partial y} \frac {\partial s}{\partial y} = \frac{-2s}{t^2} \frac{\partial t}{\partial y} I sub this back into the first equations to get \frac{\partial t}{\partial y}. However, I get a different value for...
23. ### Partial derivatives of implicitly defined functions

Sounds like a plan. So let me try this again... [-4ys^2t - 2y^2(2s\frac{\partial s}{\partial y})t - 2y^2 s^2(\frac{\partial t}{\partial y})] + [-2(\frac{\partial s}{\partial y})t^2 - 2s(2 \frac{\partial t}{\partial y})] = 1 [4ys^2t + 2y^2(2s\frac{\partial s}{\partial y})t + 2y^2...
24. ### Partial derivatives of implicitly defined functions

But how would I get the partial derivative of t wrt y from there? I mean, aren't we assuming t is a function of x and y (and with s as well?) ? So let me try this out: 8y*s^2*t + 4y^2(2 \partial s / \partial y)t + 4y^2*s^2*( \partial t / \partial y) = -7( \partial s / \partial y)*t^2 - 7s(2...
25. ### Partial derivatives of implicitly defined functions

Homework Statement If the equations x^2 - 2(y^2)(s^2)t - 2st^2 = 1 x^2 + 2(y^2)(s^2)t + 5st^2 = 1 define s and t as functions of x and y, find \partial^2 t / \partial y^2 The Attempt at a Solution Equating the two, we get 4y^2*s^2*t = -7s*t^2. My main problem is, as simple as this...
26. ### Work along a line integral

(facepalm of realization) I'd probably multiply r^3 and u together, wouldn't I. Then slap on the k at the front of the product, and equate it to F.
27. ### Work along a line integral

How would I adjust for the "cube of the distance" though ? Looking at the numerator of that unit vector, there isn't much, I think, that one can do with (x\hat{i} + y\hat{j})^3
28. ### Work along a line integral

Homework Statement A particle is attracted towards the origin by a force proportional to the cube of its distance from the origin. How much work is done in moving the particle from the origin to the point (2,4) along the path y = x^2 assuming a coefficient of friction \mu between the particle...

30. ### Surface area of a curve around the x-axis

Ah, I must've forgotten to chain rule it. y = (1/√8)√(x2 - x4) dy/dx = (1/2√8)(x2 - x4)-1/2(2x - 4x3) dy/dx = (1/√8)(x2 - x4)-1/2(x - 2x3)
31. ### Surface area of a curve around the x-axis

The factor is \sqrt{1+(dy/dx)^2}, which I'm sure I accounted for above. dr = {\sqrt{1+ ({\frac{\sqrt{2x(1-2x^2)}}{8})^2}} {dx} It might not be "dr", but I'm 99% sure that's the calculation of the factor.
32. ### Surface area of a curve around the x-axis

Homework Statement Find the surface area traced out when the curve 8y^2 = x^2(1-x^2) is revolved around the x-axis. The Attempt at a Solution x-axis means y = 0 When y = 0, x = 0, -1 or 1. Since this curve is "the infinity symbol", the curve has symmetry at x = 0. Isolating y, 8y^2 =...
33. ### Surface integral in spherical coordinates question

Yes, but unforunately, my Cartesian answer not the same. I see now why though: c\sqrt{3} \int_{-\sqrt{2c^2}}^{\sqrt{2c^2}} \int_{-\sqrt{2c^2 - x^2}}^{\sqrt{2c^2 - x^2}} ({{\sqrt{3c^2 - x^2 - y^2}})^{-{\frac{1}{2}}}}dy dx} = -c\sqrt{3} \int_{-\sqrt{2c^2}}^{\sqrt{2c^2}} ({\sqrt{3c^2 - x^2 -...
34. ### Surface integral in spherical coordinates question

But now I have a new problem with the surface area done in Cartesians. When z=c, 2c^2 = x^2 + y^2 where -\sqrt{2c^2 - x^2} \leq y \leq \sqrt{2c^2 - x^2} and -\sqrt{2c^2} \leq x \leq \sqrt{2c^2} dS for the sphere is {\sqrt{\frac{3c^2}{3c^2 - x^2 - y^2}}}dy dx Surface area =...
35. ### Surface integral in spherical coordinates question

Well, the other part of the question (which I had done already) was doing it in Cartesians, so I know the two surfaces intersect at z = c, as you stated. So, (c \sqrt{3}) cos(\varphi) = c cos (\varphi) = 1/{\sqrt{3}} \varphi = a really ugly number. Should I just keep this as cos^{-1}...
36. ### Surface integral in spherical coordinates question

Homework Statement Find the surface area of the portion of the sphere x^2 + y^2 + z^2 = 3c^2 within the paraboloid 2cz = x^2 + y^2 using spherical coordinates. (c is a constant) Homework Equations The Attempt at a Solution I converted all the x's to \rho sin\phi cos\theta, y's to \rho...
37. ### Surface integral/point charge question

That was what I was looking for, yes. >_> Because I wasn't sure if trying to get away with three separate answers would be a good idea. (One for each region.)
38. ### Surface integral/point charge question

Yes, but then it leads to my next question: are forces on the hemisphere equal to forces on the surface? (If so, I can probably just equate the surface integrals to each other. But then what happens to the hidden disk?)
39. ### Surface integral/point charge question

Anyone else have any ideas?
40. ### Surface integral/point charge question

Why would I need to find the force due to a finite circular sheet/disk though? Only 1 and 3 are the protruding surfaces.
41. ### Surface integral/point charge question

Homework Statement A uniform surface charge lies in the region z = 0 for x^2 + y^2 > a^2, and z = \sqrt{a^2-x^2-y^2} for x^2 + y^2 \leq a^2. Find the force on a unit charge placed at the point (0,0,b) Homework Equations dF = G (\delta)(vector of point to surface) / (magnitude of surface)^3...
42. ### Centroid of a triangle using Green's theorem

Because I'm only supposed to use those equations to solve the problem.
43. ### Centroid of a triangle using Green's theorem

Homework Statement Given a curve C that starts from the origin, goes to (1,0) then goes to (0,1), then back to the origin, find the centroid of the enclosed area D. Homework Equations \bar{x} = {1/(2A)}*\int_C {x^2 dy} \bar{y} = -{1/(2A)}*\int_C {y^2 dx} The Attempt at a Solution Well...
44. ### Expected value problem (probability)

Homework Statement Given Y1 and Y2 are integer values, where 0\leqY1\leq3, 0\leqY2\leq3, 1\leqY1+Y2\leq3 p(Y1, Y2) = \frac{{4 \choose y_1}{3 \choose y_2}{2 \choose {3-y_1-y_2}}}{{9 \choose 3}} Find E(Y1+Y2) and V(Y1+Y2) Homework Equations E(Y1+Y2) = E1(Y1)+E2(Y2) E1(Y1) =...