2.9 Geometry

Properties of Circles

• $r$ is the length of the radius
• $d$ is the length of the diameter
• $d=2r$
• Circumference is the perimeter of a circle. The formula for circumference is

  $\begin{array}{r}C=\frac{2}{\pi r}\end{array}$

• The formula for area of a circle is

  $A=\pi r^2$

Remember, that we approximate $\pi$ with $3.14$ or $\frac{22}{7}$ depending on whether the radius of the circle is given as a decimal or a fraction. If you use the $\pi$ key on your calculator to do the calculations in this chapter, your answers will be slightly different from the answers shown. That is because the $\pi$ key uses more than two decimal places.

Volume and Surface Area of a Sphere

For a sphere with radius $r$:

Attribution

“9.5 Solve Geometry Applications: Circles and Irregular Figures” and “9.6 Solve Geometry Applications: Volume and Surface Area” from Prealgebra 2e by Lynn Marecek, MaryAnne Anthony-Smith, and Andrea Honeycutt Mathis, © OpenStax are licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.