Question

In Exercises \(1-8,\) describe and sketch the surface. $$ 4 x^{2}+y^{2}=4 $$

Short answer

The given surface is an elliptic cylinder with semi-major axis 1 along the x-axis and semi-minor axis 2 along the y-axis.

Step-by-step solution

Simplify the equation

Divide the equation by 4, to put the right side equal to 1. \[ x^{2} + \frac{y^{2}}{4} = 1 \]

Identify the type of surface

This equation is the general format of an ellipse (in the x-y plane) with a semi-major axis of length 1, along the x-axis, and a semi-minor axis of length 2, along the y-axis. When no equation is given for z, it means the surface extends infinitely in the positive and negative z direction. Hence, this surface is a cylinder.

Sketch the surface

For editing a sketch, just draw a standard ellipse in the x-y plane and extend the ellipse infinitely in the positive and negative z-direction. The cylinder's cross section in any x-y plane is always the same ellipse.

Key concepts

Ellipse Equation

Understanding the equation of an ellipse is crucial for visualizing many types of surfaces in calculus, especially when dealing with quadratic relationships. An ellipse, simply put, is a stretched circle. It has two axes, namely the major (the longest) and the minor axis (the shortest). The general equation for an ellipse centered at the origin is given by \( \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 \) where \( a \) and \( b \) represent the lengths of the semi-major and semi-minor axes, respectively.

An important point to note is that if \( a > b \) the ellipse is stretched along the x-axis, and if \( a < b \) it stretches along the y-axis. The given exercise presents the ellipse equation \( 4x^2 + y^2 = 4 \) which, upon simplifying by dividing everything by 4, gives us \( x^2 + \frac{y^2}{4} = 1 \) indicating an ellipse with a semi-major axis of 1 and a semi-minor axis of 2.

Cylindrical Surface

When we think of a cylindrical surface, we might immediately imagine a straight, regular cylinder like a soup can. However, in calculus, a cylindrical surface can take various forms, extending infinitely in one direction. Whenever you see an equation that doesn't include one of the variables - in this case, \( z \) - you're dealing with a cylindrical surface.

The cylindrical surface represented by the ellipse equation \( x^2 + \frac{y^2}{4} = 1 \) is not your standard circular cylinder; it's an elliptical cylinder because its cross section is an ellipse, not a circle. To visualize this surface, imagine sliding the ellipse along a line perpendicular to the plane of the ellipse, in this case, the z-axis, and the surface would extend indefinitely.

Conic Sections

Conic sections are curves obtained by intersecting a cone with a plane. There are four classic types: circles, ellipses, parabolas, and hyperbolas. These shapes are created depending on the angle at which the plane intersects the cone. If the plane intersects parallel to the cone's base, a circle is obtained. An angle less steep results in an ellipse, which becomes our focus when discussing the equation from the original exercise.

An ellipse is a type of conic section where the plane's angle is less than that of the cone's side but doesn't cut through the base. The formula from the exercise, when simplified, perfectly depicts an ellipse. These conic sections are fundamental in understanding many physical phenomena, such as planetary orbits, and are also the basis for constructing certain structures, such as satellite dishes and telescopes.