Graphing 3D Equations: Solving x^2 + z^2 ≤ 9

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Homework Help Overview

The discussion revolves around graphing 3D equations, specifically focusing on the inequality x^2 + z^2 ≤ 9. Participants explore the nature of the graph and its representation in three-dimensional space.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the interpretation of the graph, with some suggesting it represents a cylinder extending along the y-axis, while others initially perceive it as a 2-D circle in the xz plane. Questions arise regarding the implications of the absence of a y term in the inequality. Additionally, there are inquiries about the representation of the graph in different octants and the labeling of axes.

Discussion Status

The discussion is active, with participants sharing their interpretations and clarifying misconceptions about the graph's representation. Some guidance has been offered regarding the nature of the graph and the significance of the y-coordinate's arbitrariness, while questions about the octants and axis labeling remain open for further exploration.

Contextual Notes

Participants note the potential confusion arising from the graph's appearance in different octants and the labeling of the axes, which may not align with standard coordinate planes. This highlights the importance of careful interpretation of graphical representations in 3D.

Miike012
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I am having difficulty graphing equatioins in 3D for instance...

what does the graph in 3-D look like x^2 + z^2 ≤ 9?

Apparently its a cylinder extending along the y-axis. My initial guess was it was a 2-D circle with radius 3 on the xz plane. Anyways how should I have known it extended along the y-axis?
 
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Miike012 said:
I am having difficulty graphing equatioins in 3D for instance...

what does the graph in 3-D look like x^2 + z^2 ≤ 9?

Apparently its a cylinder extending along the y-axis. My initial guess was it was a 2-D circle with radius 3 on the xz plane. Anyways how should I have known it extended along the y-axis?
The statement of the problem probably says something about graphing the set of points (x, y, z) such that x2 + z2 ≤ 9.

Since the inequality doesn't include a y term, the y coordinates are completely arbitrary. That's why the graph extends along the y-axis.
 
Another question... the 3d picture is on the paint doc and I am wondering why the entire surface is only in the first octant because isn't the point (which I hightligted in the paint document) in the 3rd octant?
 

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That graph is screwy.

The graph of the equation z = 2x2 + 2y2 - 4 is a paraboloid that opens up from its vertex at (0, 0, -4), and whose central axis lies along the z-axis. In your picture, the thing appears to be in the first octant.
 
Miike012 said:
Another question... the 3d picture is on the paint doc and I am wondering why the entire surface is only in the first octant because isn't the point (which I hightligted in the paint document) in the 3rd octant?

It isn't in the first octant. It is correct. The planes that look like the coordinate planes aren't ##x=0## and ##y=0##. Look how the axes are labeled.
 
LCKurtz said:
It isn't in the first octant. It is correct. The planes that look like the coordinate planes aren't ##x=0## and ##y=0##. Look how the axes are labeled.
Good point. I saw how the axes are labeled, but I didn't notice that the planes you see aren't the regular coordinate planes.
 

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