Type the circle equation and click the center. Desmos will also show a point you can use to read the radius. The algebra of circles is in circle problems. The calculator is in Desmos on the ACT.
Standard form
(x − h)² + (y − k)² = r² graphs a circle centered at (h, k) with radius r. Type it as written. Click the center point Desmos marks. If the equation is expanded, x² + y² + Dx + Ey + F = 0, you can still graph it and read the center from the figure instead of completing the square.
If nothing appears, the window is probably too small or the circle is off to the side. Zoom out. That move is in zooming when the graph is off-screen.
Do not mix this up with shading
A circle equation draws a curve. An inequality shades a region. Shading is in shading inequalities in Desmos.
The bottom line
Graph the equation, click the center, and read the radius from the equation or the figure. Zoom if the circle is missing.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.