Question 391748
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Hi,         
 hyperbola with vertices (4,0) and (-4,0) 
Opens right and left along the x-axis with center at Pt(0,0)
Standard Form of an Equation of an Hyperbola opening right and left is:
  {{{(x-h)^2/a^2 - (y-k)^2/b^2 = 1}}} where Pt(h,k) is a center  with vertices 'a' units right and left of center.
  x^2/16 - y^2 /b^2 = 1
foci at (-c,0) and (c,0) where c^2 = a^2 + b^2
foci at (sqrt(20), 0) and (sqrt(20), 0)
 sqrt(20)^2 = 16 + b^2
  20 - 16 = b^2
     b^2 = 4
       b = ± 2
   x^2/16 - y^2 /4 = 1
<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Hyperbola_properties.svg/200px-Hyperbola_properties.svg.png">