Chapter 11: Problem 52
Find an equation of the surface consisting of all points \(P(x, y, z)\) that are twice as far from the plane \(z=-1\) as from the point \((0,0,1),\) Identify the surface.
Short Answer
Expert verified
The surface is a hyperboloid of one sheet.
Step by step solution
01
Identify the Distances
To find the equation, we need to express the distances from a point \(P(x, y, z)\) to the plane \(z = -1\) and the point \((0, 0, 1)\). The distance from \(P\) to the plane is \(|z + 1|\), and the distance from \(P\) to the point \((0, 0, 1)\) is \(\sqrt{x^2 + y^2 + (z - 1)^2}\).
02
Set up the Relationship
The problem states that the distance to the plane is twice the distance to the point. Therefore, we write the equation: \[ |z + 1| = 2\sqrt{x^2 + y^2 + (z - 1)^2} \]
03
Eliminate the Absolute Value
Since \(|z + 1|\) is the absolute distance to the plane, we consider both cases: when \(z + 1 \geq 0\) and when \(z + 1 < 0\). However, since it's the absolute value of the distance, \(|z + 1| = z + 1\) can be used as is because it will simplify correctly.
04
Square Both Sides
To remove the square root, we square both sides of the equation from Step 2:\[ (z + 1)^2 = 4(x^2 + y^2 + (z - 1)^2) \]
05
Simplify the Equation
Expand both sides:- Left side: \((z + 1)^2 = z^2 + 2z + 1\)- Right side: \(4(x^2 + y^2 + z^2 - 2z + 1) = 4x^2 + 4y^2 + 4z^2 - 8z + 4\)Equating both sides gives:\[ z^2 + 2z + 1 = 4x^2 + 4y^2 + 4z^2 - 8z + 4 \]
06
Rearrange and Simplify Further
Rearrange the terms:\[ z^2 + 2z + 1 - 4z^2 + 8z - 4 = 4x^2 + 4y^2 \]Simplify:\[ -3z^2 + 10z - 3 = 4x^2 + 4y^2 \]Divide every term by \(-3\) to make it a standard form:\[ z^2 - \frac{10}{3}z + 1 = -\frac{4}{3}(x^2 + y^2) \]
07
Identify the Surface
The final equation \( z^2 - \frac{10}{3}z + 1 = -\frac{4}{3}(x^2 + y^2) \) can be identified as a hyperboloid of one sheet if we consider transformations to standard forms of quadratic surfaces based on coefficients and signs.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Distance Calculation
In this context, calculating the distance involves both a plane and a point. To determine an equation for the given surface, we must assess how far a generic point \( P(x, y, z) \) is from the specific plane \( z = -1 \) and the point \( (0, 0, 1) \).
- Distance to the Plane: The formula for the distance from a point to the plane \( z = -1 \) is given by \( |z + 1| \). This is because the plane's equation can be rewritten as \( z + 1 = 0 \), and thus the absolute value indicates the perpendicular distance in the \( z \)-axis direction.
- Distance to the Point: To calculate the distance to the point \( (0, 0, 1) \), utilize the 3D distance formula: \( \sqrt{x^2 + y^2 + (z - 1)^2} \). This expression includes the differences in each coordinate, squared, and summed under a square root, as per Pythagoras' theorem generalized in three dimensions.
Hyperboloid of One Sheet
A hyperboloid of one sheet is a fascinating quadric surface characterized by its saddle-like shape. This type of surface can be expressed in several standard forms, generally involving squares of the \( x \), \( y \), and \( z \) coordinates.
- Typically takes the form \( \frac{x^2}{a^2} + \frac{y^2}{b^2} - \frac{z^2}{c^2} = 1 \), where one variable has a negative coefficient, typifying the surface’s hyperbolic nature.
- It allows slicing through with hyperbolas and ellipses, depending on the plane of intersection, which causes the surface to appear as one continuous sheet.
- In our problem, the transformed equation resembles a hyperboloid of one sheet by having differing coefficients and a negative sign playing a crucial role in its identification.
Equations of Surfaces
Manipulating equations is crucial in identifying surfaces described by specific conditions. Transforming expressions to standard forms is one useful technique.
- Equation Setup: Initially, equations reflect the conditions set by the problem – in this case, \( |z + 1| = 2\sqrt{x^2 + y^2 + (z - 1)^2} \).
- Simplifying: Through algebraic transformations such as removing absolute values and squaring both equation sides, we can simplify complex expressions.
- Reformulating: Rewriting facilitates matching to known surface forms, such as the hyperboloid of one sheet. Adjusting the final form \( z^2 - \frac{10}{3}z + 1 = -\frac{4}{3}(x^2 + y^2) \) reveals familiar properties.