Chapter 3: Problem 46
Exer. 35-46: Find an equation of the circle that satisfies the stated conditions. $$ \text { Endpoints of a diameter } A(-5,2) \text { and } B(3,6) $$
Short Answer
Expert verified
The equation of the circle is \((x + 1)^2 + (y - 4)^2 = 20\).
Step by step solution
01
Find the Center of the Circle
The center of a circle that has a diameter as one of its line segments is the midpoint of that diameter. First, identify the coordinates of the endpoints of the diameter, which are given as \( A(-5,2) \) and \( B(3,6) \). Then calculate the midpoint using the midpoint formula: \( \text{Midpoint} = \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right) \). Substitute the known values: \( \text{Midpoint} = \left( \frac{-5 + 3}{2}, \frac{2 + 6}{2} \right) = \left( -1, 4 \right) \). Therefore, the center of the circle is \( (-1, 4) \).
02
Calculate the Radius of the Circle
The radius of the circle is half the length of the diameter. First, find the distance between the endpoints \( A(-5,2) \) and \( B(3,6) \) using the distance formula: \( d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \). This gives \( d = \sqrt{(3 + 5)^2 + (6 - 2)^2} = \sqrt{8^2 + 4^2} = \sqrt{64 + 16} = \sqrt{80} \). Therefore, the radius is \( \frac{\sqrt{80}}{2} = \sqrt{20} = 2\sqrt{5} \).
03
Write the Equation of the Circle
Now that you have the center \((-1, 4)\) and the radius \(2\sqrt{5}\), use the standard equation of a circle, \((x - h)^2 + (y - k)^2 = r^2\), where \((h, k)\) is the center and \(r\) is the radius. Substitute the known values: \((x + 1)^2 + (y - 4)^2 = (2\sqrt{5})^2\). Simplify \((2\sqrt{5})^2\) to get 20, thus the equation becomes \((x + 1)^2 + (y - 4)^2 = 20\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Midpoint Formula
To find the center of a circle when you have the endpoints of the diameter, you use the midpoint formula. This is because the midpoint of the diameter of a circle is the circle's center. The formula is:
- Midpoint: \( \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right) \)
- Midpoint: \( \left( \frac{-5 + 3}{2}, \frac{2 + 6}{2} \right) = (-1, 4) \).
Distance Formula
The distance formula is crucial for calculating the length of the diameter or any line segment in a coordinate plane. It essentially extends the Pythagorean theorem to two points in the plane. The formula is:
- Distance: \( d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \)
- \( d = \sqrt{(3 + 5)^2 + (6 - 2)^2} = \sqrt{8^2 + 4^2} = \sqrt{64 + 16} = \sqrt{80} \).
Standard Equation of a Circle
The standard equation of a circle is written using the circle's center and radius. It is expressed as:
- Equation: \( (x - h)^2 + (y - k)^2 = r^2 \)
- Center: \((-1, 4)\)
- Radius: \(2\sqrt{5}\)
- \((x + 1)^2 + (y - 4)^2 = (2\sqrt{5})^2\)
- Simplified: \((x + 1)^2 + (y - 4)^2 = 20\)
Geometry
Geometry is the mathematical study of shapes, sizes, and properties of space. A circle is a significant shape in geometry, defined as the set of all points equidistant from a fixed point, known as the center. Understanding circles involves several key concepts:
- The center is a crucial reference point, found using the midpoint of a diameter.
- The radius is the distance from the center to any point on the circle, often found using the distance formula.
- The diameter, being twice the radius, provides a full measure across the circle through the center.