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Sketching a Curve - Step by Step

We may now describe the steps in sketching a curve. We shall stick to the simple case where f and its first two derivatives are continuous on a closed interval [a, b], and either are never zero or are zero only finitely many times. (Curve plotting in a more general situation is discussed in Chapter 5 on limits.)

Step 1

Compute dy/dx and d2y/dx2.

Step 2

Find all points where dy/dx = 0 and all points where d2y/dx2 = 0.

Step 3

Pick a few points

a = x0, xl, x2, ..., xn = b

in the interval [a, b]. They should include both endpoints, all points where the first or second derivative is zero, and at least one point between any two consecutive zeros of dy/dx or d2y/dx2.

Step 4

At each of the points x0, ..., xn, compute the values of y and dy/dx and determine the sign of d2y/dx2. Make a table.

Step 5

From the table draw conclusions about where y is increasing or decreasing, where y has a local maximum or minimum, where the curve is concave upward or downward, and where it has a point of inflection. Use Theorems 1-3 of this section and the tests for maxima and minima.

Step 6

Plot the values of y and indicate slopes from the table. Then connect them with a smooth curve which agrees with the conclusions of Step 5.


Last Update: 2006-11-05