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A polynomial equation in which the highest power of the variable is 2 is called a quadratic function. We arrive at the following graph when we draw up a quadratic  function such as  y = x2: 

We can easily see that we are not dealing with a straight line but a parabola, thus it is referred to as a non-linear function.  When one has a positive coefficient before x2 we have a minimum value, and if we have a negative coefficient we have a maximum value instead. See the graph below where y = -x2:
A rule of thumb reminds us that when we have a positive symbol before x2 we get a happy expression on the graph :) and a negative symbol renders a sad expression :( .We graph our quadratic function in the same way as we graph a linear function. First we make a table for our x- and y-values. From the x values we determine our y-values.Last we graph our matching x- and y-values and draw our parabola  

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