Problem 36 - College Algebra Essentials

Question

write the standard form of the equation of the circle with the given center and radius. $$ \text { Center }(-3,5), r=3 $$

Step-by-Step Solution

Answer

The standard form of the equation of the circle with the given center and radius is \( (x + 3)^2 + (y - 5)^2 = 9 \)

Step 1: Identify given information

The information given in the exercise is the center of the circle which is (-3,5) and the radius 3.

Step 2: Substitute the center into the standard form

The x-coordinate of the center (h) is -3 and the y-coordinate (k) is 5. Substitute these values into the standard form of a circle equation, resulting in \( (x - (-3))^2 + (y - 5)^2 = r^2 \)

Step 3: Substitute the radius into the equation

The radius (r) of the circle is 3. Substitute this value into the equation obtained in step 2, resulting in \( (x - (-3))^2 + (y - 5)^2 = 3^2 \)

Step 4: Simplify the equation

Simplify the equation obtained from Step 3 by removing the double negative and square the radius, which will result in: \( (x + 3)^2 + (y - 5)^2 = 9 \)

Key Concepts

Circle Equations

Understanding the standard form of a circle's equation is a fundamental concept in geometry and crucial for solving related problems. The general equation for a circle in standard form is expressed as
\[ (x - h)^2 + (y - k)^2 = r^2 \]
where (h, k) represents the coordinates of the center of the circle and r signifies the radius. The equation basically states that any point (x, y) on the circle is exactly r units away from the center.
Using this form, it's easy to plot a circle on a coordinate plane or to determine the circle's properties, such as its diameter or area. For example, if we have a circle centered at (-3, 5) with a radius of 3, substituting these values into the standard form yields:
\[ (x - (-3))^2 + (y - 5)^2 = 3^2 \]
Simplifying the equation by squaring the radius and removing double negatives, we get the specific equation for our circle:
\[ (x + 3)^2 + (y - 5)^2 = 9 \]
This equation is very useful for graphing the circle, and it is also the starting point for finding intercepts, analyzing tangents, and solving system of equations involving circles.

Geometric Center of a Circle

In geometry, the geometric center of a circle, often referred to as the circumference's midpoint, is a crucial point called the centroid. It is denoted by its coordinates (h, k) in the equation of a circle. The center is significant because all points on the circle are equidistant from it, with that common distance being the radius.
When given a circle's equation, like \( (x + 3)^2 + (y - 5)^2 = 9 \), we can instantly determine the center by looking at the coefficients of the \( x \) and \( y \) terms within the parentheses. The center for our given circle is (-3, 5), which is essentially the point from which every line segment or radius stretches out to the circle's edge. Knowing the center is important for graphing purposes, understanding symmetry of the circle, and for calculating related geometric properties like the area or enclosing shapes.

Circle Radius

The radius of a circle is not only a line segment connecting the center of the circle to any point on its perimeter, but it also plays a central role in the circle's equation. The radius is denoted by r and appears as the value being squared on the right side of the equation, as seen in the standard form \( (x - h)^2 + (y - k)^2 = r^2 \).
In our example \( (x + 3)^2 + (y - 5)^2 = 9 \), the radius is 3 because when we take the square root of the right side of the equation, we obtain \( r = 3 \). The radius is a fundamental measure that determines the size of the circle, and it is half the length of the circle's diameter. Understanding the radius is key to solving a wide range of problems, including finding the circumference \( C = 2\text{π}r \), and the area \( A = \text{π}r^2 \), as well as solving complex problems involving arc lengths or sectors.