Problem 21 - Algebra 2 and Trigonometry

Question

In \(20-27\) : a. Write each equation in center-radius form. b. Find the coordinates of the center. . Find the radius of the circle. $$ x^{2}+y^{2}-2 x-2 y-7=0 $$

Step-by-Step Solution

Answer

Center: (1, 1); Radius: 3.

Step 1: Reorganize the Equation

Start by rearranging the original equation to group the x terms and y terms together. This will help in completing the square later. The equation becomes: \[ x^2 - 2x + y^2 - 2y - 7 = 0 \]

Step 2: Complete the Square for X

To complete the square for the \(x\) terms, take the coefficient of \(x\) which is \(-2\), divide by 2 to get \(-1\), and then square it to get \(1\). Add and subtract \(1\) inside the equation:\[ x^2 - 2x + 1 - 1 + y^2 - 2y - 7 = 0 \]

Step 3: Complete the Square for Y

Do the same for the \(y\) terms. The coefficient of \(y\) is \(-2\), so divide by 2 to get \(-1\), and then square it to get \(1\). Add and subtract \(1\) as well:\[ x^2 - 2x + 1 + y^2 - 2y + 1 - 1 - 7 = 0 \]

Step 4: Simplify and Factor

The terms can now be grouped and factored to indicate perfect squares. Combine the \(1\)s and \(-7\) on the right side:\[ (x - 1)^2 + (y - 1)^2 = 9 \]

Step 5: Identify Center and Radius

The equation is now in the center-radius form \((x - h)^2 + (y - k)^2 = r^2\). Here, \(h = 1\), \(k = 1\), and \(r^2 = 9\), so the radius \(r = 3\). Therefore, the center is \((1, 1)\) and the radius is \(3\).

Key Concepts

Completing the Square

Completing the square is a method used to transform a quadratic expression into a perfect square trinomial. This is a crucial step for converting circle equations into a more familiar format. Here's how we do it:

• First, focus on the terms involving the variable, such as the terms with "x" or "y".
• Take the coefficient of the linear term, divide it by 2, and then square the result. This helps turn the quadratic expression into a perfect square trinomial.
• Add and subtract this squared number within the equation to keep it balanced.

For example, in the equation \(x^2 - 2x\), take the coefficient of \(x\), which is \(-2\). Divide by 2 to get \(-1\), then square it to get \(1\). Adding and subtracting this 1 helps us to complete the square: \((x - 1)^2\) becomes much simpler to work with. Completing the square simplifies quadratic equations, paving the way for clearer interpretation in geometry.

Center-Radius Form

Once the square for both x and y terms are completed, the equation of the circle can be rewritten in a neat form called the center-radius form. This form clearly presents both the center of the circle and the radius. The standard center-radius form of a circle's equation is:

• \((x - h)^2 + (y - k)^2 = r^2\)

Here, \(h\) and \(k\) denote the coordinates of the circle's center, while \(r\) is the radius. In our example, after completing the square, we found:

• \((x-1)^2 + (y-1)^2 = 9\)

This indicates that the circle's center is at \((1, 1)\) and the radius is 3 (since \(r^2 = 9\), making \(r = \sqrt{9} = 3\)). The center-radius form provides a clear geometric interpretation that is essential for visualizing and understanding circles.

Equation of a Circle

The general equation for a circle in a plane is derived from its geometric definition. It represents all the points that are equidistant from a given point, called the center. A circle's equation is expressed as:

• \(x^2 + y^2 + Dx + Ey + F = 0\)

where \(D\), \(E\), and \(F\) are constants. Initially, our equation \(x^2 + y^2 - 2x - 2y - 7 = 0\) was not in a straightforward form. By rearranging and completing the square, we rewrote it into the center-radius form. This transformation helps identify critical characteristics of the circle, such as its center and radius, making the geometric properties accessible for more intuitive understanding. Identifying and converting different forms of a circle's equation is fundamental for solving geometry problems involving circles.

Geometry of Circles

Understanding the geometry of circles is crucial in many mathematical contexts, from basic geometry to advanced calculus. A circle is defined as a set of points in a plane, all equally distant from a central point. This distance is referred to as the radius. The central point is the center.

The basic geometric properties of a circle include:

• The center \( (h, k) \)
• The radius \( r \)
• A perfectly symmetric shape where every diameter measures twice the length of the radius

For our worked problem, visualizing the circle's geometry through its equation \((x - 1)^2 + (y - 1)^2 = 9\) allows us to easily understand its size and position on the coordinate plane. The center (1, 1) gives us the exact location, while the radius, 3, indicates its size. This comprehensive understanding empowers us to solve real-world problems involving circular motion, projections, and area calculations.