Tangent Lines

Suppose we have the quadratic function \(Q(x) = \frac {1}{2} (x - 2)^2 + 1\).

Then its graph is a parabola. The point \(\left ( 3, \frac {3}{2} \right )\) is on the parabola.

The line \(y=x-\frac {3}{2}\) goes through the point \(\left ( 3, \frac {3}{2} \right )\).

The parabola and line have a special relationship, which we can see by zooming in.

As you zoom in, the graph slowly looks more and more like the line.

The line does the best job of approximating the graph at the point \(\left ( 3, \frac {3}{2} \right )\).

And, this is the only line that will work for the point \(\left ( 3, \frac {3}{2} \right )\). Any other line, besides this one, will have a permanant angle between the graph and the line.

The graph and this particular line have the same “slope” at the point.

This line is called the tangent line for \(Q(x) = \frac {1}{2} (x - 2)^2 + 1\) at the point \(\left ( 3, \frac {3}{2} \right )\).

Since “slope” is a characteristic of a line and a curve is not a line, then curves do not have a slope as we know that word.

However, it is easy to see that curves have something that seems a lot like a slope at points. Curves might have different slopes at different points, but our eyes definitely see curves moving in different directions.

We want to quantize this idea of a slope for a curve at a point.

If we can quantize it, then we can build a function for it and use that to measure a rate of change for our curve.

We will first investigate this idea with quadratic functions and parabolas.