# Finding the equation of a parabola

by Flatland
Tags: equation, parabola
 P: 143 How do you find the equation of a parabola if you are given it's vertex and 1 point? For example, find the quadratic equation of a parabola that has a vertex of (-2,-2) and goes through the point (-1,0)
 P: 8 General quadratic equation: y=ax^2 + bx + c Derivative: y' = 2ax + b At the vertex, the derivate equals to zero. Use this fact and simultaneous equations to arrive at the equation.
 PF Patron Sci Advisor Thanks Emeritus P: 38,412 More simply, since you are given the vertex of the parabola, you can write the equation y= a(x-x0)2+ y0 where x0 and y0 are the coordinates of the vertex. Choose a to force the parabola to go through the second point. y= a(x-(-2))2- 2= a(x+2)2- 2. Setting x= -1, y= 0, 0= a(-1+2)2-2= a- 2 so a= 2. By the way, this is assuming the parabola has a vertical line of symmetry. Otherwise there are an infinite number of parabolas satisfying these conditions.
P: 37

## Finding the equation of a parabola

 Quote by HallsofIvy ... assuming the parabola has a vertical line of symmetry...
Graph first, and you may find a shortcut for a given specific data.
If a point on a parabola is 1 to the right and 2 up from its vertex, it must be parabola
$$y = 2x^{2}$$
shifted horizontally and vertically, so its vertex (0,0) moves into (-2,-2), i.e. 2 to the left and 2 down:
$$y = 2(x+2)^{2} - 2$$

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