Variable Definition and 1000 Threads

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

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  1. J

    Heisenberg's Uncertainty or hidden variable

    Intuitively it seems more plausible that there should be a hidden variable influencing the path of particles when they travel, than having a fundamental mechanism of nature be truly random, but what do I know. I just don't understand how you can ever completely rule out a hidden variable. I know...
  2. P

    MHB Expected Value of Negative Binomial Random Variable

    Given $X$ as a negative binomial random variable with parameters $r$ and $p$. Find $E(\frac{r-1}{X-1})$. As $E(g(X))$ is defined as $\sum_{x\in X(\Omega)}g(x)p(x)$, this is my attempt in which I am stuck. What can I do next? In the case $y=r-1$, is the sum invalid? Thanks in advance!
  3. B

    Variable change in definite integration

    I am having trouble understanding how the author of the textbook "Physics for scientists and engineers" (Randall D Knight), is performing the variable change in the definite integral of the picture that I have taken a screen clipping of. Can someone please help me figure this out?
  4. Q

    Trying to solve complex equation with 1 variable,

    Homework Statement Guys, could you please help me solve this equation for x: 220000=\sqrt{1.4*10^{32}*\left[ln(\frac{6.17*10^{20}+x}{6.17*10^{20}})-\frac{x}{6.17*10^{20}+x}\right]*\frac{1}{x}} It's supposed to give me the radius of the Milky Way's dark matter halo. I expect to get a...
  5. E

    Is it possible to create a metal ring with a variable diameter using segments?

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  6. N

    Statistics: variable transformation proof?

    Homework Statement Ok this might be a stupid question, but: https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-frc3/t31/q77/s720x720/10001118_10202561443653973_1625797585_o.jpg Why is this the case? I think for all of this to be right, then the assumption of ##Y=u(X) \Leftrightarrow...
  7. A

    Amplifing variable 0-5 v dc voltage to 0-10 voltge range

    i am middle of engg project . to make an automatic irrigation system ,using plc , main problem is humidity sensor which i am using gives out put of 0-5 volt but plc require more output for better results,...can anyone suggest a voltage amplifier or any circuit using op amp
  8. L

    Derivative of a function whose variable is a matrix

    Homework Statement Let ##f: M_{n \times n} \rightarrow M_{n \times n}## with ##f(X) = X^2##, where ##M_{n \times n}## denotes the vector space of ##n \times n## matrices. Show ##f## is differentiable and find its differential. Homework Equations The Attempt at a Solution So far...
  9. S

    Projectile Motion Question To Find Unknown Variable (Initial Velocity)

    Hey all! I've tried solving this problem every way I could think of and I wanted to see if I'm doing this even remotely right. Mind helping me out? Homework Statement Problem: A mountain climber is stranded on a ledge 30 m above the ground. Rescuers on the ground want to shoot a projectile to...
  10. J

    MHB Problem solving 2 variable equal triangle problem

    I solved this many years ago, but after revisiting Trig in order to tutor my daughter, I revisited this to stimulate myself but am hitting a brick wall. Problem: A 4 inch square sits in a corner(picture x,y origin). A 12 inch ruler or line leans against the wall at an angle such that there are...
  11. M

    How to Understand the Change of Variable in a Vectorial Function Integral?

    Hi there, I got across the integral $$\int_{\omega} \nabla y(x) \mathrm{d}x$$. It would be better to perform the integration over the domain $$\Omega$$, the two domains being related by a transformation $$Y:\omega \to \Omega$$. Using the change of variable rule I wrote $$\int_{\Omega}...
  12. S

    Probablity: What's the p.d.f. of the random variable Z = X|X|

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  13. T

    Optimising 2 variable s to maximise a third

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  14. M

    Ansys workbench-spring with variable stiffness

    Hello every body, I am simulating a 1 dof spring-mass model in ansys workbench. I need to change the stiffness of spring at every substep based on an "if condition". It means that, in every substep, if a specified condition is satisfied, the stiffness will be changed to a defined value. Also...
  15. J

    Changing variable in summation

    Like in the integration, exist a formula to compute the summation by parts, that is: \frac{\Delta }{\Delta x}(f(x)g(x))=\frac{\Delta f}{\Delta x}g+f\frac{\Delta g}{\Delta x}+\frac{\Delta f}{\Delta x}\frac{\Delta g}{\Delta x}\sum \frac{\Delta }{\Delta x}(f(x)g(x))\Delta x = \sum \frac{\Delta...
  16. J

    Can't Identify Variable in QM Question

    Hello, my question is a simple one: I am attempting to do problem 2.31 in Griffiths' QM book (latest edition). The question states, "The Dirac Delta function can be thought of as the limiting case of a rectangle of area 1, as the height goes to infinity and the width to zero. Show that the...
  17. Borek

    Java Coding in Java - replacement for a global variable

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  18. S

    Conditional distribution for random variable on interval

    Homework Statement Find the conditional distribution function and density for the random variable X defined on R given that X is in some interval I = (a,b) where P(X in I) > 0. Assume that the density and distribution for the random variable X is known Homework Equations fX|X\inI =...
  19. P

    Solving System of Linear Equations: x_2 Free Variable?

    Homework Statement |1 0-1-2-8 | -3| |0 0 1 2 7 | 1| |0 0 0 1 -4 | -13| I think that x_{2} is a free variable, so when writing the general solution in a solution set, what would I put for it? Nothing?
  20. B

    Determining Which Variable Is Independent

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  21. U

    Solving Unknown Variables in Homework Problems

    Homework Statement The problem is attached. Homework Equations The Attempt at a Solution I've been trying to take the Log for both sides but the problem is I have (1+0.05X) if I just have e^-0.1 X, I will take Log for both sides and solve it but in this case I have (1+0.05X) too...
  22. alyafey22

    MHB Solutions of symmetric three variable equations.

    Inspired by this http://mathhelpboards.com/challenge-questions-puzzles-28/solving-system-equations-8521.htmlit is interesting to look at the solutions of xz+y=a xy+z=a zy+x=a where we look at the case a>0 Assume the following a=bc where a\neq 0 Then we have If we assume that c is a...
  23. J

    Ferrite core variable inductor

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  24. F

    Why Does Changing the Direction of Δv Affect the Rocket Equation Results?

    I'm having some issues with the rocket equation. I'm deriving the velocity as a function of time for a descending rocket (so the rocket is accelerating upwards in order to slow the descent). The result I should obtain is v=v0+gt+uln(mf/mi) where mf is the final mass, mi is the initial...
  25. H

    Solving Variable Velocity Water Pouring Problem with Calculus

    I've been relearning c++ and my friend gave me a problem to try and solve programmatically. Here is the set up: There is a pitcher with a gallon of water. The water is being poured out beginning at a rate of 2x milliliters per second. x begins at 1 and increases by 1 over 5 seconds. The...
  26. jk22

    Proof that no non-local variable can exist ?

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  27. C

    MHB Intersection Points & Finding Unknown Variable

    The line with equation y = x + k, where k is a real number, intersects the parabola with equation y = x^2 + x − 2 in two distinct points if I first made the equations equal each other x + k = x^2 + x − 2 0 = x^2 -2 -k From here i thought you use the discriminate a=1 b=o c=-2-k but this...
  28. P

    Solving for a variable, transposing equation

    I have an equation, where acceleration is affected by a driving power and air resistance. The acceleration is given by: a = (2P / m + u^2)^0.5 - (k*p*A*u^2 / 2m) - u I'm trying to make "u" the subject, which is previous velocity, to find at what velocity does acceleration become 0, the...
  29. Y

    MHB Bivariate discrete random variable

    Hello I am trying to solve this problem: A coin is given with probability 1/3 for head (H) and 2/3 for tail (T). The coin is being drawn N times, where N is a Poisson random variable with E(N)=1. The drawing of the coin and N are independent. Let X be the number of heads (H) in the N draws...
  30. Lebombo

    Change in domain as variable changes in Parametric Equations.

    This question is from the parametric equations chapter of my calc book. I am given x =\frac{1}{\sqrt{t+1}} and y = \frac{t}{t+1}, for (x> -1) Solving the x(t) for t, we get \frac{1- x^{2}}{x^{2}} Eliminating the parameters by substitution, we get y = 1 - x^{2} for (x > 0) My question is...
  31. D

    Probabilty with random variable

    Homework Statement A couple is expecting the arrival of a new boy. They are deciding on a name from the list S = { Steve, Stanley, Joseph, Elija }. Let X(ω) = first letter in name. Find Pr(X = S). Homework Equations The Attempt at a Solution Ok the answer is 2/3. How is it 2/3...
  32. N

    Simple C programming problem, variable stuck at initialised value

    Simple C programming problem, variable "stuck" at initialised value Hello! This is most likely an error in my code but I just can't see what it is! I am new to C programming and maybe I have just been staring at it for so long that I can't see the obvious error. I was playing around with a...
  33. M

    Finding the minimum of a 2 variable function.

    Homework Statement ind the absolute minimum value of the function f(x,y) = 6 + 3xy -2x- 4y on the set D. D is bounded by the parabola y=x^2 and y=4 Homework Equations Partial derivatives and a theorem where if F is continuous on a closed bounded set in R(2) then F has an abs. max and...
  34. J

    Limits of Integration Variable

    Knowing that the limits of integration of a any function, for example: \int_{-\infty}^{+\infty}\delta (x)dx=1 I know that's correct call your primitive through the limit superior as a variable, so H(x)=\int_{-\infty}^{x}\delta (x)dx But, and if I want to describe your primitive through the...
  35. X

    Where can I have custom made a variable dc switching power supply?

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  36. F

    Double Integral Choosing Change of Variable

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  37. D

    F is continuous then F is continuous in each variable separately

    Homework Statement Let F: X x Y -> Z. We say F is continuous in each variable separately if for each ##b \in Y## the function h: X -> Z, h(x) = F(x,b), and for each ##a \in X##, the function g: Y -> Z, g(y) = F(a,y) is continuous. Show that if F is continuous, then F is continuous in each...
  38. B

    Can anyone explain how to store a string as a variable?

    I don't have any big programs. I just write programs to get me through my math classes. But I want to have the output look pretty. Like one program I have is for my finance class. prompt p if p = 1 then Prompt b,r,n,t r/100→x b/(1+x/n)^(n*t)→l disp l else Prompt b,r,n,t r/100 →...
  39. A

    Do you integrate a variable charge density when solving for Q?

    For example, let's say there was an block infinitely long in the x and y direction and in the z direction bounded by positive and negative a. I am trying to find the charge of a imaginary partition infinitely long in the x and y direction and in the z direction bounded by positive and...
  40. S

    Basic Statistics: Reciprocal Variable Better Fit?

    Hello, I have some data that I am looking into correlation factors (Pearson). Let's say I have two data sets: X and Y. I have Graph X vs. Y then graph 1/X vs. Y. The reciprocal (1/X) yields a more linear relationship. Why does this occur? Thanks in advance!
  41. S

    One Variable at a Time Approach

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  42. M

    Why do we say the upper limit is the variable?

    While doing work, I always see notation like: F(x) = ∫0xf(t)dt - first equation F(x) = ∫1xf(t)dt What is the exact reasoning we always use the upper limit? What is the lower limit was a variable? From FTC 2, the answer for the first equation is F(x) - F(0) right yet is written as F(x) when...
  43. J

    Derivatives with multiple variable, help

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  44. S

    Center of Mass w/ variable density

    Homework Statement A box with one corner at the origin and the opposite corner at (a,b,c). The density is:##\rho=\rho_o\frac{z^2}{c^2}## Homework Equations ##r_{cm}=\frac{\int\vec{r}dm}{\int dm}## The Attempt at a Solution I calculated the mass...
  45. R

    3 variable process controls - I have no idea

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  46. M

    Multiple Random Variable Question

    Homework Statement A and B agree to meet at a certain place between 1 PM and 2 PM. Suppose they arrive at the meeting place independently and randomly during the hour. find the distribution of the length of time that A waits for B. (If B arrives before A, define A's waiting time as...
  47. G

    Raindrop Velocity and Mass Dynamics: Finding Space Traversed Over Time

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  48. N

    MHB Canceling fractions when more than 1 variable in an equation

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  49. I

    Moment generating function, CDF and density of a random variable

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  50. M

    Work Energy Theorem for Variable Mass Systems

    How do we know that the work-energy theorem holds for variable mass systems? Or rather, since I'm sure that we can at least know it to be true experimentally, what is the mathematical basis for the work-energy theorem? I know for fixed mass systems, a rather simple derivation comes from...
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