Chapter 10: Problem 25
Classify each of the following as the equation of either a circle, an ellipse, a parabola, or a hyperbola. $$ x^{2}+y^{2}-6 x+10 y-40=0 $$
Short Answer
Expert verified
Circle
Step by step solution
01
Identify the Type of Conic Section
Compare the given equation to the general forms of a circle, ellipse, parabola, and hyperbola. Notice that both x and y are squared in the equation.
02
Complete the Square for x and y
To classify the conic section, rewrite the equation by completing the square. Start by grouping the x terms and the y terms together: \[ x^2 - 6x + y^2 + 10y = 40 \]
03
Complete the Square for x
Add and subtract the same value to complete the square for the x terms: \[ x^2 - 6x + 9 - 9 \] This simplifies to \[ (x-3)^2 - 9 \]
04
Complete the Square for y
Add and subtract the same value to complete the square for the y terms: \[ y^2 + 10y + 25 - 25 \] This simplifies to \[ (y+5)^2 - 25 \]
05
Substitute Completed Squares Back into the Equation
Substitute these modified terms back into the equation to obtain: \[ (x-3)^2 - 9 + (y+5)^2 - 25 = 40 \]
06
Simplify the Equation
Combine and simplify all the constants on one side: \[ (x-3)^2 + (y+5)^2 - 34 = 40 \] \[ (x-3)^2 + (y+5)^2 = 74 \]
07
Identify the Resulting Conic Section
The equation now matches the general form of a circle: \[ (x-h)^2 + (y-k)^2 = r^2 \] with center \((h, k) = (3, -5)\) and radius \(r = \sqrt{74}\). Hence, it is a circle.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Equation of a Circle
Understanding the equation of a circle is crucial when working with conic sections. The general form of a circle's equation is \((x - h)^2 + (y - k)^2 = r^2\). Here, \(h\) and \(k\) represent the coordinates of the circle's center, and \(r\) represents the radius.
For instance, if you come across an equation like \((x - 3)^2 + (y + 5)^2 = 74\), you can identify it as a circle with:
Notice how the equation is structured:
For instance, if you come across an equation like \((x - 3)^2 + (y + 5)^2 = 74\), you can identify it as a circle with:
- Center at \( (3, -5) \)
- Radius \( \sqrt{74} \)
Notice how the equation is structured:
- Each variable, \(x\) and \(y\), is squared.
- There are no additional x or y terms outside the parentheses.
- All terms are on one side of the equation, with a constant on the other side.
Completing the Square
Completing the square is an important algebraic technique used to convert quadratic equations into a more recognisable form. Here's how it works:
2. Add and subtract this square inside the equation to form a perfect square trinomial: \( x^2 - 6x + 9 - 9 = (x-3)^2 - 9 \).
- Suppose we have \ x^2 - 6x \. To complete the square, follow these steps:
2. Add and subtract this square inside the equation to form a perfect square trinomial: \( x^2 - 6x + 9 - 9 = (x-3)^2 - 9 \).
- Do the same for the \(y\) terms. Adding and subtracting \(25\), derived similarly, converts \ y^2 + 10y \ to \( (y+5)^2 - 25 \).
Identifying Conic Sections
When you are given a quadratic equation in two variables, identifying the type of conic section it represents is essential. Conic sections (circle, ellipse, parabola, hyperbola) can be recognized by the following characteristics:
For the equation \ x^2 + y^2 - 6x + 10y - 40 = 0 \, we see both \(x^2\) and \(y^2\) terms with the same coefficients (which are both 1 here):
- By completing the square, it simplifies the equation into the form of a circle,
- Through these steps, each equation can be classified into its respective conic section. So, knowing the general forms and applying techniques like completing the square is key.
- A parabola has only one squared term, either \(x^2\) or \(y^2\), but not both.
- An ellipse and a circle both have \(x^2\) and \(y^2\) with the same sign, but a circle's coefficients of the squared terms are equal.
- A hyperbola has \(x^2\) and \(y^2\) with opposite signs.
For the equation \ x^2 + y^2 - 6x + 10y - 40 = 0 \, we see both \(x^2\) and \(y^2\) terms with the same coefficients (which are both 1 here):
- By completing the square, it simplifies the equation into the form of a circle,
- Through these steps, each equation can be classified into its respective conic section. So, knowing the general forms and applying techniques like completing the square is key.