10217

# Two Lines Determine a Circle

Two lines rotate at the same speed around separate centers. Surprisingly, their intersection always outlines a circle that passes through the two pivot points. Their phase relation, indicated by the blue wedge, determines the size of the circle. A 90 degree offset puts the two pivot points at each end of a diameter.

### DETAILS

To understand why this phenomenon is so, notice that the light blue point of intersection goes around the circle twice for every single 360° rotation of the lines. This is just another way of observing that the central angle of an arc of a circle is twice any angle drawn from any point on the circle itself. The two red pivot points can now be seen as two such points on a circle.

### PERMANENT CITATION

 Share: Embed Interactive Demonstration New! Just copy and paste this snippet of JavaScript code into your website or blog to put the live Demonstration on your site. More details » Download Demonstration as CDF » Download Author Code »(preview ») Files require Wolfram CDF Player or Mathematica.

#### Related Curriculum Standards

US Common Core State Standards, Mathematics

 RELATED RESOURCES
 The #1 tool for creating Demonstrations and anything technical. Explore anything with the first computational knowledge engine. The web's most extensive mathematics resource. An app for every course—right in the palm of your hand. Read our views on math,science, and technology. The format that makes Demonstrations (and any information) easy to share and interact with. Programs & resources for educators, schools & students. Join the initiative for modernizing math education. Walk through homework problems one step at a time, with hints to help along the way. Unlimited random practice problems and answers with built-in Step-by-step solutions. Practice online or make a printable study sheet. Knowledge-based programming for everyone.