Back in the day, curve sketching by hand was an important part of precalculus. But with the advent of the graphing calculator, sketching curves by hand isn’t usually necessary any more. Graphing calculators are allowed on most calculus exams (even AP Calculus), so you can graph your function on the TI-89 to get an idea of the overall shape. Then it’s just a matter of labeling the curve.
You may be asked to specifically include:
- Horizontal asymptotes,
- Vertical asymptotes,
- Critical Points (maximums and minimums),
- Points of inflection,
Read on for detailed information about how to find each of these important graph parts.
1. Curve Sketching: Finding X-intercepts
X-intercepts are where the function crosses the x-axis. These points are called roots or zeros. There are many ways to find roots, including on the TI-89 and using the rational number theorem. See the main roots/zeros article for more details on how to solve for a function’s roots.
You can also find roots for curve sketching with the quadratic formula. This requires you to have some strong algebra skills (including the ability to recognize when you can use the quadratic formula, and when you cannot).
Example question: Find the zeros of this function algebraically: f(x) = x2 – 10x + 16
- Set f(x) to zero: 0 = x2 – 10x + 16,
- Identify a, b, and c. A quadratic function, like this one, has the form ax2 + bx + c = 0. So, a = 1, b = -10, c = 16.
- Plug a, b, and c values into the quadratic formula:
- Solve, using algebra, to get two roots: 8 and 2.
If you’re algebra is a little rusty, check out Symbolab’s calculator, which shows the steps for this solution.
2. Finding Y-intercepts
The next important topic in curve sketching: you’ll want to find where your graph crosses the Y-axis. To find the y-intercept, set x to zero and solve.
For example: y = x2 + 5x
Set x to zero and solve: y = 02 + 5(0) = 0
This function’s y-intercept is at x = 0.
For a quadratic equation, the y-intercept is the point “c”:
y = ax2 + bx + c
For example, y = 10x2 + 5x + 9, the y-intercept is 9.
In calculus, you won’t see more than one y intercept, because it creates a major issue called “one to many.”
3. Horizontal asymptotes
4. Vertical asymptotes
5. Critical Points (maximums and minimums)
6. Points of inflection
See: Points of inflection
Curve Sketching: References
Khan, D. Cracking the AP Calculus AB & BC Exams. Random House Information Group. 2009.