Preliminary reading: Bryant ch. 4, Courant and John ch. 2.
Concurrent reading: Hart, Spivak ch. 9 and ch. 10.
Further reading: Mason, Tall (1982).
Definition of derivative
It is easy enough to say when a straight line is a tangent to a circle or an ellipse. For these curves, a straight line – infinitely extended – meets the curve in 0, 1 or 2 points. Each line with a unique point of intersection is a tangent. However, if we try to use such a test to identify tangents to other curves we are in for a disappointment, and on several counts.
1 At how many points does the line x = 1 intersect the parabola y = x2? Draw a sketch. Is this line a tangent to the curve?
2 At how many points does the line y = −2 intersect the cubic curve y = x3 − 3x? Draw a sketch. Is this line a tangent to the curve?
From qn 2 we learn that whether a line is a tangent to a curve or not is a local question, which must be asked relative to the particular point of intersection, that is, inside a sufficiently small neighbourhood of that point.
3 At how many points does the line y = mx intersect the cubic curve y = x3? For how many values of m might the line be a tangent to the curve?
Check that the line y = h2x is the chord joining the two points (0, 0) and (h, h3) on the curve, provided h ≠ 0.
If h → 0, to what does the slope (or gradient) of the chord tend?
4 Write down the equation of the line with slope m through the point(a, f(a)).
Write down the equation of the chord joining the points (a, f(a)) and (a + h, f(a + h)).