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Chapter 12 Conics. Section 2 Circles. Section 12.2 Objectives. 1 Write the Standard Form of the Equation of a Circle 2 Graph a Circle 3 Find the Center and Radius of a Circle from an Equation in General Form. Axis. Axis. Axis. Axis. Parabola. Ellipse. Circle. Hyperbola.
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Chapter 12 Conics Section 2 Circles
Section 12.2 Objectives 1 Write the Standard Form of the Equation of a Circle 2 Graph a Circle 3 Find the Center and Radius of a Circle from an Equation in General Form
Axis Axis Axis Axis Parabola Ellipse Circle Hyperbola Conic Sections Conics, an abbreviation for conic sectionsare curves that result from the intersection of a right circular cone and a plane. The four conics are shown below.
r (h, k) (x, y) Radius and Center • Acircleis a set of all points in the Cartesian plain that are a fixed distance r from a fixed point (h, k). The fixed distance r is called the radius, and the fixed point (h, k) is called the center of the circle.
Standard Form of a Circle The standard form of an equation of a circle with radius r and center (h, k) is (x – h)2 + (y – k)2 = r2. Example: Determine the equation of the circle with radius 4 and center (– 5, 2). (x – h)2 + (y – k)2 = r2 center (– 5, 2) r = 4 (x – (– 5))2 + (y – (2))2 = 42 (x + 5)2 + (y – 2)2 = 16
y 8 (– 5, 6) 6 4 (– 5, 2) 2 (– 1, 2) (– 9, 2) x 8 8 6 4 2 2 4 6 (– 5, – 2) 4 6 8 Graphing a Circle Example: Graph the equation (x + 5)2 + (y – 2)2 = 16. h = – 5, k = 2 r = 4 The center is (– 5, 2). The radius is 4.
General Form of a Circle The general form of the equation of a circle is given by the equation x2 + y2 + ax + by + c = 0 when the graph exists. Example: Determine the equation of the circle: x2 + y2 + 2x – 8y + 8 = 0 Regroup the terms. (x2 + 2x) + (y2 – 8y) = – 8 (x2 + 2x + 1) + (y2 – 8y + 16) = – 8 + 1 + 16 Complete the square in both x and y. (x+ 1)2 + (y – 4)2 = 9 Factor.