All lessons › Unit 6 · Geometry and trigonometry

Circle equations

A circle equation like (x − 3)² + (y + 2)² = 25 tells you the center and radius right away. The Digital SAT Math section often gives the equation already expanded and asks for the center or the radius.

This lesson starts with the standard form and how to read the center and radius. Then it teaches completing the square, which turns an expanded equation back into one you can read. Each part has a quiz, and the practice questions give you expanded equations to rewrite.

In this lesson

• The circle equation
• Completing the square