title: Mathematica: How to use a single equation with multiple parameters to calculate any parameter
body: <p>Currently I use a single equation with different combination of known/unknown parameters. As I don't have any fancy calculator it would be much easier to define the equation in Mathematica and passing known parameters to calculate unknown values.</p>
  
  <p>I would be very thankful if anyone of you could give an example solution (if possible using given equation).</p>
  
  <p>Let's say we have an equation of satellite speed at given point in the elliptical orbit:</p>
  
  <p>v = sqrt(u(2/r - 1/a))</p>
  
  <p>where </p>
  
  <p>v = speed
  u = constant 3.986 * 10^14 m^3/s^2
  r = radius (distance from the center of the Earth)
  a = semi major axis of the ellipse</p>
  
  <p>This equation can be used to calculate the speed or for example we know what is the speed needed for a manoeuvre to move the cargo to other orbit and have to model the orbit (a) at given radius (r)</p>
  
  <p>Thanks!</p>