Chapter 9.5, Problem 36E

To determine

To calculate: To plot the point in rectangular coordinates and find two setsof polar representations of the point, using $0<\theta <2\pi$

Answer to Problem 36E

The rectangular coordinates of point on the graph is plotted and two sets of polar points are $\left(3\sqrt{2},\frac{5\pi }{4}\right),\left(-3\sqrt{2},\frac{\pi }{4}\right)$

Explanation of Solution

Given information: Rectangularis $\left(-3,-3\right)$

Formula Used:

Polar coordinate of a point $\left(x,y\right)$ is given as

  $\left(r,\theta \right)$

Where,

  $r=\sqrt{x^2+y^2}$

  $\theta =\arctan \left(\left|\frac{y}{x}\right|\right)$

If both $x,y>0$ , then $\theta =\alpha$

If $x<0,y>0$ , then $\theta =\pi -\alpha$

If $x,y<0$ , then $\theta =\pi +\alpha$

If $x>0,y<0$ , then $\theta =-\alpha$

Point $\left(r,\theta \right)$ can be represented as

  $\left(r,\theta \right)=\left(r,\theta \pm 2n\pi \right)$

Or

  $\left(r,\theta \right)=\left(-r,\theta \pm \left(2n+1\right)\pi \right)$

Calculation:

Rectangular point is given as

  $\left(-3,-3\right)$

Plotting the rectangular coordinates on the graph

  

Converting rectangular coordinates into polar coordinates:

Polar coordinate of a point $\left(x,y\right)$ is given as

  $\left(r,\theta \right)$

Calculating the value of $r$ :

  $\begin{array}{l}r=\sqrt{{\left(-3\right)}^2+{\left(-3\right)}^2}\\ r=\sqrt{9+9}\\ r=\sqrt{18}\\ r=3\sqrt{2}\end{array}$

Calculating the value of $\theta$ :

Since $x,y<0$ , then $\theta =\pi +\alpha$

  $\begin{array}{l}\theta =\pi \arctan \left(\left|\frac{-3}{-3}\right|\right)\\ \theta =\pi +\frac{\pi }{4}\\ \theta =\frac{5\pi }{4}\end{array}$

Thus, polar coordinate of point is $\left(3\sqrt{2},\frac{5\pi }{4}\right)$

Now, let’s find another set polar representations of the point, where $0<\theta <2\pi$

Point $\left(r,\theta \right)$ can be represented as

  $\left(r,\theta \right)=\left(-r,\theta \pm \left(2n+1\right)\pi \right)$

Here,

  $\begin{array}{l}r=3\sqrt{2}\\ \theta =\frac{5\pi }{4}\end{array}$

Substituting the values in above representations, another representation of given point are:

  $\left(-3\sqrt{2},\frac{5\pi }{4}-\pi \right)=\left(-3\sqrt{2},\frac{\pi }{4}\right)$

Conclusion:

Hence, rectangular coordinates of point on the graph is plotted and twosets of polar points are $\left(3\sqrt{2},\frac{5\pi }{4}\right),\left(-3\sqrt{2},\frac{\pi }{4}\right)$