How to Use a Graphing Calculator to Make a Circle

More advanced graphing calculators can graph a circle directly.

Graphing calculators have all sorts of little limitations that make them simultaneously easier and harder to use. A great example of this is trying to use a graphing calculator to draw a circle. You'll notice that you can't simply enter the equation for a circle into a calculator's graphing program. That's because the equation for a circle isn't a function in the mathematical sense when written in standard form. However, you can turn it into one by doing a little bit of algebra.

Instructions

1. Find the dependent variable in the equation for your circle. For example, in the circle X^2 + Y^2 = 1, "Y" acts as the dependent variable.

2. Move all terms except for "Y" over to the other side via addition and subtraction. The circle X^2 + Y^2 = 1 becomes Y^2 = 1 - X^2.

3. Take the square root of both sides. Note that you end up with two answers when you take a square root. The circle becomes Y = ±√(1 - X^2).

4. Enter the equations into the graphing menu of your calculator. You should enter two equations, Y = √(1 - X^2) and Y = -√(1 - X^2).

5. Press the "Graph" key. The calculator will graph a circle.

References

• Texas Instruments: Graphing a Circle on a TI Graphing Calculator