9.1 Introduction to Circles

Some definitions you need Circle – set of all points in a plane that are equidistant from a given point called a center of the circle. A circle with center P is called “circle P”, or P. P

Some definitions you need A radius is a segment whose endpoints are the center of the circle and a point on the circle. QP, QR, and QS are radii of Q. All radii of a circle are congruent. Ignore the PS

Some definitions you need A chord is a segment whose endpoints are points on the circle. A secant is a line that contains a chord (or basically it intersects the circle at two points) A diameter is a chord that passes through the center of the circle.

Some definitions you need A tangent is a line in the plane of a circle that intersects the circle in exactly one point, called the point of tangency.

Sphere A sphere with center ) and radius r is the set of all points in space at a distance r from point O. Can you determine which line is the radius? Secant? Tangent? Chord?

Ex. 1: Identifying Special Segments and Lines Tell whether the line or segment is best described as a chord, a secant, a tangent, a diameter, or a radius of C. a.AD b.CD c.EG d.HB

Ex. 1: Identifying Special Segments and Lines a.AD – Diameter because it contains the center C. b.CD – radius because C is the center and D is a point on the circle. c.EG – a tangent because it intersects the circle in one point. d.HB - is a chord because its endpoints are on the circle.

First Assignment Stop and do page Classroom Exercises. Then continue the notes. Remember: The diameter is twice the radius. The radius is half the diameter.

9.2 Tangents to Circles Goal: Apply definitions and theorems about tangents to figures to calculate segment lengths Purpose: We are studying this material because tangents are segments that provide missing information about lengths in circles.

Theorem 9.1 If a line is tangent to a circle, then it is perpendicular to the radius drawn to the point of tangency. If l is tangent to Q at point P, then l ⊥ QP. l

Definition: Common Tangents – a line that is common to two circles Common internal tangents Common external tangents

Definition: Tangent Circles – circles that are tangent to the same line at the same point These two circles are externally tangent These two circles are internally tangent

Corollary If two tangents originate from the same point, then the tangents are congruent

Ex: Using properties of tangents AB is tangent to C at B. AD is tangent to C at D. Find the value of x. x 2 + 2

Solution: x = x Two tangent segments from the same point are  Substitute values AB = AD 9 = x 2 Subtract 2 from each side. 3 = xFind the square root of 9. The value of x is 3 or -3.

Ex: Verifying a Tangent to a Circle You can use the Converse of the Pythagorean Theorem to tell whether EF is tangent to D. Because 11 2 _ 60 2 = 61 2, ∆DEF is a right triangle and DE is perpendicular to EF. So by Theorem 10.2; EF is tangent to D.

Second Assignment Stop and do page Written Exercises Hand in both assignments to me