Slope of a curve

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Two young mathematicians discuss the novel idea of the ‘‘slope of a curve.’’

Check out this dialogue between two calculus students (based on a true story):

Devyn: Riley, do you remember ‘‘slope?’
Riley: Most definitely. ‘‘Rise over run.’’
Devyn: You know it.
Riley: ‘‘Change in $y$ over change in $x$ .’’
Devyn: That’s right.
Riley: Brought to you by the letter ‘‘ $m$ .’’
Devyn: Enough! My important question is: could we define ‘‘slope’’ for a curve that’s not a straight line?
Riley: Well, maybe if we ‘‘zoom in’’ on a curve, it would look like a line, and then we could call it ‘‘the slope at that point.’’
Devyn: Ah! And this ‘‘zoom in’’ idea sounds like a limit!
Riley: This is so awesome. We just made math!

The concept introduced above, of the ‘‘slope of a curve at a point,’’ is in fact one of the central concepts of calculus. It will, of course, be completely explained. Let’s explore Devyn and Riley’s ideas a little more, first.

To find the ‘‘slope of a curve at a point,’’ Devyn and Riley spoke of ‘‘zooming in’’ on a curve until it looks like a line. When you zoom in on a smooth curve, it will eventually look like a line. This line is called the tangent line. It is the line tangent to $f(x)$ at $x=a$ .

Which of the following approximate the slope of the ‘‘zoomed line’’? Select all that are correct.

$\frac {(f(a)+h) - f(a)}{(a+h)-a}$ $\frac {f(a+h) - f(a)}{(a+h)-a}$ $\frac {(f(a)-h) - f(a)}{(a-h)-a}$ $\frac {f(a-h) - f(a)}{(a-h)-a}$ $\frac {f(a) - (f(a)+h)}{a-(a+h)}$ $\frac {f(a) - f(a+h)}{a-(a+h)}$ $\frac {f(a) - (f(a)-h)}{a-(a-h)}$ $\frac {f(a) - f(a-h)}{a-(a-h)}$

Let $f(x) = 3x-1$ . Zoom in on the curve around $a = -2$ so that $h = 0.1$ . Use one of the formulations in the problem above to approximate the slope of the curve. The slope of the curve at $a = -2$ is approximately… $\answer {3}$

Repeat the previous problem for $f(x) = x^2 - 1$ , $a = 0$ , and $h = 0.2$ . Choose a formulation that will give you a positive answer for the slope. The (positive) slope of the curve at $a = 0$ is approximately… $\answer {0.2}$

Zoom in on the curve $f(x) = x^2 - 1$ near $x=0$ again. By looking at the graph, what is your best guess for the actual slope of the curve at zero?

impossible to say zero one infinity