Vertex equations of a quadratic function and it's inverse

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A parabola can be uniquely defined by its vertex V=(v_x, v_y) and one more point P=(p_x, p_y). The function term of the parabola then has the form

y = a (x-v_x)^2 + v_y.

a can be determined by solving

p_y = a (p_x-v_x)^2 + v_y for a which gives

a = (p_y - v_y) / (p_x - v_x)^2 .


JavaScript code

var b = JXG.JSXGraph.initBoard('box1', {boundingbox: [-5, 5, 5, -5], grid:true});
var v = b.create('point', [0,0], {name:'V'}),
    p = b.create('point', [3,3], {name:'P'}),
    f = b.create('functiongraph', [
             function(x) {
                 var den = p.X()- v.X(),
                     a = (p.Y() - v.Y()) / (den * den);
                 return a * (x - v.X()) * (x - v.X()) + v.Y();
             }]);

})();

Inverse quadratic function

Conversely, also the inverse quadratic function can be uniquely defined by its vertex V and one more point P. The function term of the inverse function has the form

y = sqrt((x-v_x)/a) + v_y.

a can be determined by solving

p_y = sqrt((p_x-v_x)/a) + v_y for a which gives

a = (p_x - v_x) / (p_y - v_y)^2 .


JavaScript code

var b = JXG.JSXGraph.initBoard('box2', {boundingbox: [-5, 5, 5, -5], grid:true});
var v = b.create('point', [0,0], {name:'V'}),
    p = b.create('point', [3,3], {name:'P'}),
    f = b.create('functiongraph', [
             function(x) {
                 var den = p.Y()- v.Y(),
                     a = (p.X() - v.X()) / (den * den),
                     sign = (p.Y() >= 0) ? 1 : -1;
                 return sign * Math.sqrt((x - v.X()) / a) + v.Y();
             }]);