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# CIRCLE - PowerPoint PPT Presentation

CIRCLE. Analytic Geometry Ms. Charmeigne Geil A. Abalos August 23, 2012. Terminology:. A Circle is the set of all points in a plane equidistant from a fixed point. The fixed point is called the center and the positive constant equal distance is called the radius of the circle.

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### CIRCLE

Analytic Geometry

Ms. CharmeigneGeil A. Abalos

August 23, 2012

A Circle is the set of all points in a plane equidistant from a fixed point.

The fixed point is called the center and the positive constant equal distance is called the radius of the circle.

An equation of the circle can be obtained if its center, C(h,k), and the radius are known.

The circle having center at the point C(h,k) and the radius r has an equation

(x-h)2 + (y-k) 2 = r2

Illustration: let P(x,y) be a point on the circle

C(h,k)

P(x,y)

lPCl = r

Hence,

(x-h)2 + (y-k) 2 = r

Squaring both sides of the equation, we obtain

(x-h)2 + (y-k) 2 = r2

This equation is called the center-radius or the standard form of an equation of a circle with center, C(h,k) and radius r.

Now, if we remove the parentheses of the center-radius form of an equation of the circle, then we obtain

x2 – 2hx + h2 + y2 – 2ky + k2 = r2

x2 + y2 – 2hx – 2ky + (h2 + k2 – r2) = 0

Let D=-2 of an equation of the circle, then we obtainh, E=-2k, and F= h2 + k2 – r2. By substitution, the equation becomes

x2 + y2+ Dx + Ey + F = 0

This equation is called the general form of an equation of a circle.

Theorem 2.29 of an equation of the circle, then we obtain

Let A, B, C, and D be real numbers such that A=0. Then the graph of the equation

Ax2 + Ay2 + Bx + Cy + D = 0 is a circle, a point, or the empty set. This equation can be transformed to the standard form

(x-h)2 + (y-k) 2 = q

Remarks: of an equation of the circle, then we obtain

• If q=0, then the only solution to the equation is the point (h,k). Thus, the graph of the equation Ax2 + Ay2 + Bx + Cy + D = 0 is a point.

• If q>0, then the equation can be written as (x-h)2 + (y-k) 2 =( q )2. This is the standard form of an equation

of the circle with the center C( of an equation of the circle, then we obtainh,k) and the radius r= q . Therefore, the graph of the equation Ax2 + Ay2 + Bx + Cy + D = 0 is a circle.

(c) If q<0, then (x-h)2 + (y-k) 2 = q has no solution because the sum (x-h)2 + (y-k) 2 is at least zero. Thus, the graph Ax2 + Ay2 + Bx + Cy + D = 0 is the empty set.

Observe the following examples: of an equation of the circle, then we obtain

• Find the general equation of the circle with C(-2, 3) and radius r=4.

Solution:

using the standard form (x-h)2 + (y-k) 2 = r2 with h=-2, k=3, and r=4, we obtain

(x-2)2 + (y-3) 2 = 42

(x2 + 4x + 4) + (y2 – 6y + 9) =16

x2 + y2 + 4x – 6y – 3 = 0

Answer: x2 + y2 + 4x – 6y – 3 = 0

Using the standard form (x- C(5,-2) and the circle passes through the point P(-1,5).h)2 + (y-k) 2 = r2 , we have

(x-5)2 + (y+2) 2 = ( 85 ) 2

x2 – 10x + 25 + y2 + 4y + 4 = 85

x2 + y2 – 10x + 4y – 56 = 0