Sal proves that the radius that connects the intersection point of a tangent line with the circle is perpendicular to the tangent line. Watch the next lesson: https://www.khanacademy.org/math/high-school-math/math2/math2-circles/math2-tangents/v/segments-tangent-to-circle-from-outside-point-congruent?utm_source=YT&utm_medium=Desc&utm_campaign=highschoolmath Missed the previous lesson? https://www.khanacademy.org/math/high-school-math/math2/math2-circles/math2-inscribed-shapes/v/example-showing-supplementary-oppositie-angles-in-inscribed-quadrilateral?utm_source=YT&utm_medium=Desc&utm_campaign=highschoolmath High School Math on Khan Academy: Did you realize that the word "algebra" comes from Arabic (just like "algorithm" and "al jazeera" and "Aladdin")? And what is so great about algebra anyway? This tutorial doesn't explore algebra so much as it introduces the history and ideas that underpin it. About Khan Academy: Khan Academy is a nonprofit with a mission to provide a free, world-class education for anyone, anywhere. We believe learners of all ages should have unlimited access to free educational content they can master at their own pace. We use intelligent software, deep data analytics and intuitive user interfaces to help students and teachers around the world. Our resources cover preschool through early college education, including math, biology, chemistry, physics, economics, finance, history, grammar and more. We offer free personalized SAT test prep in partnership with the test developer, the College Board. Khan Academy has been translated into dozens of languages, and 100 million people use our platform worldwide every year. For more information, visit www.khanacademy.org, join us on Facebook or follow us on Twitter at @khanacademy. And remember, you can learn anything. ... https://www.youtube.com/watch?v=St3MU5i3chc Note: All Khan Academy content is available for free at (www.khanacademy.org)