## 17Calculus - Converting Between Rectangular and Polar Coordinates

##### 17Calculus

On this page, we discuss converting between polar and rectangular (cartesian) coordinates in two main areas, converting points and converting equations.

Equations

As discussed on the main polar coordinates page, the two main equations we use to convert between polar and rectangular (cartesian) coordinates are

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

All other equations are derived from these two equations and the equation of a circle with radius $$r$$, $$x^2+y^2=r^2$$.

This short video clip explains really well how to use trigonometry and the Pythagorean Theorem to get these equations.

### PatrickJMT - Converting Between Polar and Rectangular (Cartesian) Coordinates [50secs]

video by PatrickJMT

Rectangular → Polar Points

In this case, we are given a specific point $$(x,y)$$ and we want to find $$r$$ and $$\theta$$ that describes this point. This direction is a little more complicated because we need to pay special attention to the signs of $$x$$ and y.

The equations we use are $$r = \sqrt{x^2+y^2}$$ and $$\theta= \arctan(y/x)$$. Let's discuss these equations in more detail. This discussion is critical for you to understand in order to correctly determine the polar coordinates. The first equation looks easy but there is a hidden assumption that you need to be aware of. Looking at the graph on the right, you know from the Pythagorean Theorem that $$r^2 = x^2 + y^2$$. So far, so good. Now, to solve for $$r$$ we take the square root of both sides.

$$\begin{array}{rcl} r^2 & = & x^2 + y^2 \\ \sqrt{r^2} & = & \pm \sqrt{x^2 + y^2} \\ r & = & \pm \sqrt{x^2 + y^2} \end{array}$$

In the second line above, we could have put the $$\pm$$ sign on either side, but to be strictly true, we needed it. (Do you see why?)

Now, in the equation $$r=\sqrt{x^2+y^2}$$ we dropped the $$\pm$$ sign. What does that mean? That means we are assuming that $$r$$ is always positive. So, when we convert from rectangular to polar coordinates, we will take $$r$$ to be positive. This is a subtle point but you need to keep that in mind. (As a teacher, one of my favorite questions on homework or exams will be to ask what happens when $$r$$ is negative. See the practice problems below for examples of this case.)

Okay, now for the equation $$\theta= \arctan(y/x)$$. This takes special care. You cannot just divide y by x and plug that value into your calculator. You need to know the sign of both x and y, which will determine the quadrant of your answer and thus the angle $$\theta$$. For discussion here, we want the angle to be in the interval $$(-\pi, \pi]$$. Notice this is a half-open (or half-closed) interval and includes $$\pi$$ but excludes $$-\pi$$. Of course, you need to check with your instructor to see what they require.

The best way that we've found to determine this angle, can be found using these equations.

determining $$\theta$$ for $$\tan(\theta)=y/x$$

$$\theta = \arctan(y/x)$$

$$x > 0$$

quadrant 1 or 4

$$\theta = \arctan(y/x)$$ $$+\pi$$

$$x < 0, y \geq 0$$

$$\theta = \arctan(y/x)$$ $$-\pi$$

$$x < 0, y < 0$$

$$\theta = +\pi/2$$

$$x = 0, y > 0$$

$$\theta = -\pi/2$$

$$x = 0, y < 0$$

$$\theta = 0$$

$$x=0, y=0$$

Okay, time for some practice problems.

Practice

given $$(x,y)=(0,2)$$; calculate $$(r,\theta)$$

Problem Statement

given $$(x,y)=(0,2)$$; calculate $$(r,\theta)$$

$$(r,\theta) = (2,\pi/2)$$

Problem Statement

given $$(x,y)=(0,2)$$; calculate $$(r,\theta)$$

Solution

### PatrickJMT - 753 video solution

video by PatrickJMT

$$(r,\theta) = (2,\pi/2)$$

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given $$(x,y)=(-3,4)$$; calculate $$(r,\theta)$$

Problem Statement

given $$(x,y)=(-3,4)$$; calculate $$(r,\theta)$$

$$(r,\theta) = (5,2.215)$$

Problem Statement

given $$(x,y)=(-3,4)$$; calculate $$(r,\theta)$$

Solution

### PatrickJMT - 754 video solution

video by PatrickJMT

$$(r,\theta) = (5,2.215)$$

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given $$(x,y)=(1,-5)$$; calculate $$(r,\theta)$$

Problem Statement

given $$(x,y)=(1,-5)$$; calculate $$(r,\theta)$$

$$(r,\theta) = (\sqrt{26},-1.373)$$

Problem Statement

given $$(x,y)=(1,-5)$$; calculate $$(r,\theta)$$

Solution

### 755 video solution

$$(r,\theta) = (\sqrt{26},-1.373)$$

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Rectangular → Polar Equations

In this case, we are given an equation $$f(x,y)=0$$ and we need to find an equation $$g(r, \theta)=0$$. This is pretty easy. We just substitute for x and y using these equations and simplify.

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

Practice

Basic

given $$x^2+y^2=1$$; calculate $$g(r,\theta)=0$$

Problem Statement

given $$x^2+y^2=1$$; calculate $$g(r,\theta)=0$$

$$r-1 = 0$$

Problem Statement

given $$x^2+y^2=1$$; calculate $$g(r,\theta)=0$$

Solution

### 762 video solution

$$r-1 = 0$$

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given $$y=2x+1$$; calculate $$g(r,\theta)=0$$

Problem Statement

given $$y=2x+1$$; calculate $$g(r,\theta)=0$$

$$\displaystyle{ r + \frac{1}{2\cos(\theta)-\sin(\theta)} = 0 }$$

Problem Statement

given $$y=2x+1$$; calculate $$g(r,\theta)=0$$

Solution

### 763 video solution

$$\displaystyle{ r + \frac{1}{2\cos(\theta)-\sin(\theta)} = 0 }$$

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given $$y=3/x$$; calculate $$g(r,\theta)=0$$

Problem Statement

given $$y=3/x$$; calculate $$g(r,\theta)=0$$

$$\displaystyle{ r^2 - \frac{3}{\cos(\theta)\sin(\theta)} = 0 }$$

Problem Statement

given $$y=3/x$$; calculate $$g(r,\theta)=0$$

Solution

### 764 video solution

$$\displaystyle{ r^2 - \frac{3}{\cos(\theta)\sin(\theta)} = 0 }$$

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given $$y=x^2$$; calculate $$g(r,\theta)=0$$

Problem Statement

given $$y=x^2$$; calculate $$g(r,\theta)=0$$

$$r - \tan(\theta)\sec(\theta) = 0$$

Problem Statement

given $$y=x^2$$; calculate $$g(r,\theta)=0$$

Solution

### 771 video solution

$$r - \tan(\theta)\sec(\theta) = 0$$

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given $$xy=1$$; calculate $$g(r,\theta)=0$$

Problem Statement

given $$xy=1$$; calculate $$g(r,\theta)=0$$

$$r^2 - \sec(\theta)\csc(\theta) = 0$$

Problem Statement

given $$xy=1$$; calculate $$g(r,\theta)=0$$

Solution

### 772 video solution

$$r^2 - \sec(\theta)\csc(\theta) = 0$$

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Intermediate

For the two 3-dim curves, $$2z=x^2-y^2+2x$$ and $$3z=4x^2+y^2-2x$$, determine the projection of the intersection of these curves in the xy-plane and describe the projection in polar coordinates.

Problem Statement

For the two 3-dim curves, $$2z=x^2-y^2+2x$$ and $$3z=4x^2+y^2-2x$$, determine the projection of the intersection of these curves in the xy-plane and describe the projection in polar coordinates.

Hint

In the xy-plane, the z coordinate is zero.

Problem Statement

For the two 3-dim curves, $$2z=x^2-y^2+2x$$ and $$3z=4x^2+y^2-2x$$, determine the projection of the intersection of these curves in the xy-plane and describe the projection in polar coordinates.

Hint

In the xy-plane, the z coordinate is zero.

Solution

### 1764 video solution

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Polar → Rectangular Points

In this case, we are given specific $$r$$ and $$\theta$$ values and we want to find the point $$(x,y)$$. This is the easiest direction because we already have the equations in the form we need, i.e. $$x = r \cos(\theta)$$ and $$y = r \sin(\theta)$$.

Practice

Basic

calculate $$(x,y)$$; given $$(r,\theta)=(4,\pi/3)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(4,\pi/3)$$

$$(x,y) = (2,2\sqrt{3})$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(4,\pi/3)$$

Solution

### 756 video solution

$$(x,y) = (2,2\sqrt{3})$$

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calculate $$(x,y)$$; given $$(r,\theta)=(2,\pi)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(2,\pi)$$

$$(x,y) = (-2,0)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(2,\pi)$$

Solution

### 757 video solution

$$(x,y) = (-2,0)$$

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calculate $$(x,y)$$; given $$(r,\theta)=(1,\pi/4)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(1,\pi/4)$$

$$(\sqrt{2}/2, \sqrt{2}/2)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(1,\pi/4)$$

Solution

### 765 video solution

$$(\sqrt{2}/2, \sqrt{2}/2)$$

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calculate $$(x,y)$$; given $$(r,\theta)=(-2,2\pi/3)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(-2,2\pi/3)$$

$$(1, -\sqrt{3})$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(-2,2\pi/3)$$

Solution

### 766 video solution

$$(1, -\sqrt{3})$$

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calculate $$(x,y)$$; given $$(r,\theta)=(1,-\pi/3)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(1,-\pi/3)$$

$$(1/2, -\sqrt{3}/2)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(1,-\pi/3)$$

Solution

### 767 video solution

$$(1/2, -\sqrt{3}/2)$$

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calculate $$(x,y)$$; given $$(r,\theta)=(3,3\pi/2)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(3,3\pi/2)$$

$$(0, -3)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(3,3\pi/2)$$

Solution

### 768 video solution

$$(0, -3)$$

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calculate $$(x,y)$$; given $$(r,\theta)=(2,-\pi/4)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(2,-\pi/4)$$

$$(\sqrt{2}, -\sqrt{2})$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(2,-\pi/4)$$

Solution

### 769 video solution

$$(\sqrt{2}, -\sqrt{2})$$

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Intermediate

calculate $$(x,y)$$; given $$(r,\theta)=(-7,-2\pi/3)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(-7,-2\pi/3)$$

$$(x,y) = (7/2, 7\sqrt{3}/2)$$

Problem Statement

calculate $$(x,y)$$; given $$(r,\theta)=(-7,-2\pi/3)$$

Solution

### 758 video solution

$$(x,y) = (7/2, 7\sqrt{3}/2)$$

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Polar → Rectangular Equations

Equations - - Here, we are given a function $$g(r,\theta)=0$$ and we need to find an equation $$f(x,y)=0$$. One of the keys to solving these problems is to use the identity $$\cos^2(\theta) + \sin^2(\theta) = 1$$. In terms of the above equations, we have
$$\begin{array}{rcl} x^2 + y^2 & = & [ r \cos(\theta) ]^2 + [ r \sin(\theta) ]^2 \\ & = & r^2[ \cos^2(\theta) + r \sin^2(\theta)] \\ & = & r^2 \end{array}$$

Practice

Basic

calculate $$f(x,y)=0$$; given $$r=3\cos(\theta)$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=3\cos(\theta)$$

$$x^2 - 3x + y^2 = 0$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=3\cos(\theta)$$

Solution

### 759 video solution

$$x^2 - 3x + y^2 = 0$$

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calculate $$f(x,y)=0$$; given $$r=-5\cos(\theta)$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=-5\cos(\theta)$$

$$x^2 + 5x + y^2 = 0$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=-5\cos(\theta)$$

Solution

### 770 video solution

$$x^2 + 5x + y^2 = 0$$

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Intermediate

calculate $$f(x,y)=0$$; given $$r^2=-3\sec(\theta)$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r^2=-3\sec(\theta)$$

$$x^4 + x^2y^2 - 9 = 0$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r^2=-3\sec(\theta)$$

Solution

### 760 video solution

$$x^4 + x^2y^2 - 9 = 0$$

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calculate $$f(x,y)=0$$; given $$r=3\sin(\theta)\tan(\theta)$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=3\sin(\theta)\tan(\theta)$$

$$x^3 + xy^2 - 3y^2 = 0$$

Problem Statement

calculate $$f(x,y)=0$$; given $$r=3\sin(\theta)\tan(\theta)$$

Solution

### 761 video solution

$$x^3 + xy^2 - 3y^2 = 0$$

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