Chapter 2: Problem 340
For the following exercises, the equation of a quadric surface is given. a. Use the method of completing the square to write the equation in standard form. b. Identify the surface. $$ 4 x^{2}-y^{2}+z^{2}-8 x+2 y+2 z+3=0 $$
Short Answer
Expert verified
The surface is a hyperboloid of one sheet.
Step by step solution
01
Group and rearrange terms
Start by grouping the quadratic terms and linear terms together for each variable in the equation: \[ 4x^{2} - 8x - y^{2} + 2y + z^{2} + 2z + 3 = 0 \] Rearrange it to: \[ (4x^{2} - 8x) - (y^{2} - 2y) + (z^{2} + 2z) + 3 = 0 \]
02
Complete the square for x-term
Take the x-term \(4x^{2} - 8x\). Factor out 4: \[ 4(x^{2} - 2x) \] To complete the square, take the coefficient of \(x\), halve it and square it: \[ (-2/2)^{2} = 1 \] Add and subtract 1 inside the bracket: \[ 4(x^{2} - 2x + 1 - 1) = 4((x - 1)^{2} - 1) = 4(x - 1)^{2} - 4 \]
03
Complete the square for y-term
Next, complete the square for \(-y^{2} + 2y\). Factor out \(-1\): \[ -(y^{2} - 2y) \] Take the coefficient of \(y\), halve it, and square it: \[ (2/2)^{2} = 1 \] Add and subtract 1 inside the bracket: \[ -(y^{2} - 2y + 1 - 1) = -( (y - 1)^{2} - 1) = -(y - 1)^{2} + 1 \]
04
Complete the square for z-term
Now complete the square for \(z^{2} + 2z\): Take the coefficient of \(z\), halve it, and square it: \[ (2/2)^{2} = 1 \] Add and subtract 1: \[ (z^{2} + 2z + 1 - 1) = ( (z + 1)^{2} - 1) = (z + 1)^{2} - 1 \]
05
Substitute back into the equation
Substitute the completed square forms back into the equation: \[ 4(x-1)^{2} - 4 - (y-1)^{2} + 1 + (z+1)^{2} - 1 + 3 = 0 \] Combine all constant terms: \[ 4(x-1)^{2} - (y-1)^{2} + (z+1)^{2} - 1 = 0 \] Simplify to the standard form: \[ 4(x-1)^{2} - (y-1)^{2} + (z+1)^{2} = 1 \]
06
Identify the quadric surface
The equation \[ 4(x-1)^2 - (y-1)^2 + (z+1)^2 = 1 \] is the standard form for an ellipsoid, hyperboloid of one sheet, or hyperboloid of two sheets. Check the signs and compare with the standard forms: This equation corresponds to a hyperboloid of one sheet since there are two positive squared terms and one negative squared term.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Completing the Square
Completing the square is a technique used to transform quadratic equations into a more manageable form, allowing easier analysis and solving. This method is especially useful in the context of quadric surfaces, where it helps to convert a complex equation into its standard form.
Here's a quick summary of how you complete the square for a quadratic expression containing variables like \(x\), \(y\), or \(z\):
Here's a quick summary of how you complete the square for a quadratic expression containing variables like \(x\), \(y\), or \(z\):
- Identify the quadratic and linear coefficients in each term.
- Factor out the leading coefficient if it's not 1.
- Take the coefficient of the linear term, halve it, and square the result.
- Add and subtract this square inside the parentheses, effectively creating a perfect square trinomial.
- Adjust the equation by adding or subtracting this value outside the squared term to balance the equation.
Standard Form of Quadric Surfaces
Quadric surfaces are three-dimensional analogs of conic sections, described by second-degree polynomial equations in three variables. Converting their equations to standard form makes it easier to identify their type and visualize their geometry.
The standard form of a quadric surface can generally be written as:
\[ Ax^2 + By^2 + Cz^2 + D = 1 \]
where the coefficients \(A\), \(B\), and \(C\) determine the type and orientation of the surface.
The standard form of a quadric surface can generally be written as:
\[ Ax^2 + By^2 + Cz^2 + D = 1 \]
where the coefficients \(A\), \(B\), and \(C\) determine the type and orientation of the surface.
- If all coefficients are of the same sign, it's an ellipsoid.
- If two coefficients are positive and one is negative, it's a hyperboloid of one sheet.
- If two coefficients are negative and one is positive, it's a hyperboloid of two sheets.
- A coefficient of zero would lead to a cylinder or paraboloid.
Hyperboloid of One Sheet
A hyperboloid of one sheet is a fascinating surface in three-dimensional space, emerging from specific quadratic equations. This surface resembles a smooth, elongated hourglass or a squashed cylinder, depending on the perspective.
The general form of its equation is:
\[ rac{(x-h)^2}{a^2} + rac{(z-k)^2}{c^2} - rac{(y-l)^2}{b^2} = 1 \]
which highlights its structure:
The general form of its equation is:
\[ rac{(x-h)^2}{a^2} + rac{(z-k)^2}{c^2} - rac{(y-l)^2}{b^2} = 1 \]
which highlights its structure:
- Two positive terms indicate the presence of an axis of symmetry along those planes.
- A negative term indicates a saddle-like curvature, where the surface bends in opposite directions.