Problem 41 - Contemporary Precalculus

Question

Find a rectangular equation that is equivalent to the given polar equation. $$r=3$$

Step-by-Step Solution

Answer

Question: Convert the polar equation $$r = 3$$ into a rectangular equation. Answer: The rectangular equation equivalent to the polar equation $$r = 3$$ is $$x^2 + y^2 = 9$$.

Step 1: Substitute the polar coordinates with rectangular coordinates

We have the polar equation $$r = 3$$. Let's substitute the polar coordinates with rectangular coordinates using the formulas: $$x = r\cos\theta$$ $$y = r\sin\theta$$ But first, we need to eliminate $$r$$ to get an equation in terms of $$x$$ and $$y$$. We can accomplish this using the Pythagorean identity $$r^2 = x^2+y^2$$. So, in our case, we have: $$r^2 = 9$$ Now, we can substitute $$x^2+y^2$$ for $$r^2$$.

Step 2: Write the rectangular equation

Now that we have $$r^2 = 9$$, we can write the rectangular equation as: $$x^2 + y^2 = 9$$ This is the equivalent rectangular equation for the given polar equation $$r = 3$$.

Key Concepts

Polar Coordinates

Polar coordinates are a way to represent points in a plane using a distance and an angle. They are different from the usual rectangular (or Cartesian) coordinates, which use two perpendicular axes, typically labeled as the x-axis and y-axis, with distances measured from a central point called the origin. In polar coordinates, a point is described by:

• An angle \(\theta\), measured from a fixed direction (often the positive x-axis)
• A distance \(r\) from the origin

Think of it like telling someone where to go by saying 'go this far in that direction.' This method is especially useful in scenarios involving rotation or where symmetry is more naturally aligned with a circular or radial arrangement.
For example, consider drawing a circle: it can be much simpler to describe points along the circumference using angles and a constant radius rather than horizontal and vertical distances.

Polar to Rectangular Conversion

Converting polar coordinates to rectangular coordinates means translating a location described by an angle and radius into one described by \(x\) and \(y\) coordinates. This can be essential in many situations, such as transforming complex equations into a more usable form.The conversion is done using the following relationships:

• \(x = r\cos\theta\)
• \(y = r\sin\theta\)

These equations come from trigonometry: if you drop a perpendicular from your point (\(r, \theta\)) to the x-axis, you create a right triangle where \(r\) is the hypotenuse, and the other two sides are \(x\) and \(y\).
This method allows you to transform positions and equations to match the coordinate system that best suits the problem at hand, like when you wish to integrate, differentiate, or solve the geometry within rectangular coordinates.

Pythagorean Identity

The Pythagorean identity is a fundamental relationship in trigonometry derived from the Pythagorean theorem. It relates the squares of the sine and cosine of angles:\[\sin^2\theta + \cos^2\theta = 1\]This identity is incredibly useful when dealing with conversions between polar and rectangular coordinates because it underlies the relationship between \(r\), \(x\), and \(y\). Specifically, the identity echoes the fundamental nature of a circle, which is where this identity holds true.To apply it in conversions, consider the squared form involving distances in rectangular coordinates:\[x^2 + y^2 = r^2\]In this, \(x = r\cos\theta\) and \(y = r\sin\theta\); thus, substituting and simplifying using the identity, you inevitably arrive at transformations consistent with circle equations in rectangular terms.
This framework allows mathematicians to seamlessly convert circular and radial information into linear coordinates—and vice versa—facilitating a broad range of calculations and problem-solving techniques.