HOW TO GRAPH A PARABOLA IN VERTEX FORM

Vertex form equation of a parabola :  

y  =  a(x - h)2 + k

Characteristics of graph :

The vertex is (h, k).

The axis of symmetry is x = h.

The graph opens up if a > 0 and opens down if a < 0.

Example 1 : 

Graph : y = -½(x + 3)2 + 4

Solution : 

Equation of the parabola is in vertex form : 

y = a(x - h)2 + k

a = -½, h = -3, and k = 4

Because a < 0, the parabola opens down.

To graph the function, first plot the vertex (h, k) = (-3, 4).

Draw the axis of symmetry x = -3. 

Plot two points on one side of it, such as (-1, 2) and (1, -4). Use symmetry to plot two more points, such as (-5, 2) and (-7, -4). 

Use symmetry to complete the graph.

Example 2 : 

Graph :  y = (x - 3)2 + 2

Solution : 

Equation of the parabola is in vertex form : 

y = a(x - h)2 + k

a = 1, h = 3, and k = 2

Because a < 0, the parabola opens down.

To graph the function, first plot the vertex (h, k) = (3, 2).

Draw the axis of symmetry x = 3

Plot two points on one side of it, such as (1, 3) and (1, 6). Use symmetry to plot two more points, such as (4, 3) and (5, 6).

Use symmetry to complete the graph.

Example 3 : 

Graph : y = -(x - 3)2 + 2

Solution : 

Equation of the parabola is in vertex form : 

y = a(x - h)2 + k

a = -1, h = 3, and k = 2

Because a < 0, the parabola opens down.

To graph the function, first plot the vertex (h, k) = (3, 2).

Draw the axis of symmetry x = 3. 

Plot two points on one side of it, such as (2, 1) and (1, -2).  Use symmetry to plot two more points, such as (4, 1) and (5, -2).

Use symmetry to complete the graph.

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