AP Calculus BC : Polar Form

Study concepts, example questions & explanations for AP Calculus BC

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Polar Form

Rewrite in polar form:

\(\displaystyle x^{2} = 3xy + 1\)

Possible Answers:

\(\displaystyle r =\sqrt{ \frac{1}{\cos \theta + 3 \sin \theta\; } }\)

\(\displaystyle r =\sqrt{ \frac{1}{\cos ^{2} \theta + 3 \cos \theta\; \sin \theta }}\)

\(\displaystyle r =\sqrt{ \frac{1}{\sin \theta + 3 \cos \theta\; } }\)

\(\displaystyle r =\sqrt{ \frac{1}{\cos ^{2} \theta - 3 \cos \theta\; \sin \theta }}\)

\(\displaystyle r = \sqrt{\frac{1}{\sin ^{2} \theta - 3 \cos \theta\; \sin \theta }}\)

Correct answer:

\(\displaystyle r =\sqrt{ \frac{1}{\cos ^{2} \theta - 3 \cos \theta\; \sin \theta }}\)

Explanation:

\(\displaystyle x^{2} = 3xy + 1\)

\(\displaystyle \left (r \cos \theta \right ) ^{2} = 3\left (r \cos \theta \right )\left (r \sin \theta \right ) + 1\)

\(\displaystyle r ^{2} \cos ^{2} \theta = 3 r^{2} \cos \theta\; \sin \theta \right ) + 1\)

\(\displaystyle r ^{2} \cos ^{2} \theta - 3 r^{2} \cos \theta\; \sin \theta \right ) = 1\)

\(\displaystyle r ^{2} \left ( \ \cos ^{2} \theta - 3 \cos \theta\; \sin \theta \right ) = 1\)

\(\displaystyle r ^{2} = \frac{1}{\cos ^{2} \theta - 3 \cos \theta\; \sin \theta }\)

\(\displaystyle r =\sqrt{ \frac{1}{\cos ^{2} \theta - 3 \cos \theta\; \sin \theta }}\)

Example Question #11 : Functions, Graphs, And Limits

What is the following coordinate in polar form?

\(\displaystyle (-2,-5)\)

Provide the angle in degrees.

Possible Answers:

\(\displaystyle (\sqrt{29},248)\)

\(\displaystyle (10,248)\)

\(\displaystyle (\sqrt{29},68)\)

\(\displaystyle (16,112)\)

\(\displaystyle (\sqrt{29},202)\)

Correct answer:

\(\displaystyle (\sqrt{29},248)\)

Explanation:

To calculate the polar coordinate, use

\(\displaystyle r=\sqrt{x^2+y^2}\)

\(\displaystyle r=\sqrt{29}\)

\(\displaystyle \theta=tan^{-1}\left(\frac{y}{x}\right)=68\)

However, keep track of the angle here. 68 degree is the mathematical equivalent of the expression, but we know the point (-2,-5) is in the 3rd quadrant, so we have to add 180 to it to get 248.

Some calculators might already have provided you with the correct answer.

\(\displaystyle \theta=248\).

Example Question #2 : Polar Form

What is the equation \(\displaystyle y=2x^{2}\) in polar form?

Possible Answers:

\(\displaystyle r=\frac{1}{2}\cos \theta}{\tan \theta\)

\(\displaystyle r=\frac{1}{2}\tan \theta}{\sec \theta\)

\(\displaystyle r=\frac{1}{2}\sin \theta}{\tan \theta\)

\(\displaystyle r=\frac{1}{2}\sin \theta}{\cos \theta\)

\(\displaystyle r=\frac{1}{2}\sin \theta}{\sec \theta\)

Correct answer:

\(\displaystyle r=\frac{1}{2}\tan \theta}{\sec \theta\)

Explanation:

We can convert from rectangular form to polar form by using the following identities: \(\displaystyle y=r\sin \theta\) and \(\displaystyle x=r\cos \theta\). Given \(\displaystyle y=2x^{2}\), then \(\displaystyle r\sin \theta=2(r\cos \theta )^{2}=2r^{2}\cos^{2} \theta\).

\(\displaystyle r\sin \theta=2(r\cos \theta )^{2}=2r^{2}\cos^{2} \theta\). Dividing both sides by \(\displaystyle r\cos \theta\),

\(\displaystyle \tan \theta=2r\cos \theta\)

\(\displaystyle \frac{\tan \theta}{\cos \theta}=2r\)

\(\displaystyle \tan \theta}{\sec \theta=2r\)

\(\displaystyle \frac{1}{2}\tan \theta}{\sec \theta=r\)

 

Example Question #202 : Ap Calculus Bc

What is the equation \(\displaystyle y=\frac{6}{x}\) in polar form?

Possible Answers:

\(\displaystyle r=\sqrt\frac{6}{\sin \theta cos \theta}\)

\(\displaystyle r=\sqrt\frac{6}{\cos \theta}\)

\(\displaystyle r=\sqrt\frac{6}{\sin \theta}\)

None of the above

\(\displaystyle r=\sqrt\frac{6}{\tan \theta}\)

Correct answer:

\(\displaystyle r=\sqrt\frac{6}{\sin \theta cos \theta}\)

Explanation:

We can convert from rectangular form to polar form by using the following identities: \(\displaystyle y=r\sin \theta\) and \(\displaystyle x=r\cos \theta\). Given \(\displaystyle y=\frac{6}{x}\), then \(\displaystyle r\sin \theta=\frac{6}{r\cos \theta}\). Multiplying both sides by \(\displaystyle r\cos \theta\),

\(\displaystyle r^{2}\sin \theta cos \theta=6\)

\(\displaystyle r^{2}=\frac{6}{\sin \theta cos \theta}\)

\(\displaystyle r=\sqrt\frac{6}{\sin \theta cos \theta}\)

Example Question #1 : Polar Form

Convert the following function into polar form:

\(\displaystyle f(x)=2x^2+5x+2+2y^2\)

Possible Answers:

\(\displaystyle r^2+1+5r\cos(\theta)\)

\(\displaystyle r^2+\sin(\theta)\)

\(\displaystyle 2r^2+5r\sin(\theta)+2\)

\(\displaystyle 2r^2+5r\cos(\theta )+2\)

Correct answer:

\(\displaystyle 2r^2+5r\cos(\theta )+2\)

Explanation:

The following formulas were used to convert the function from polar to Cartestian coordinates:

\(\displaystyle x=r\cos(\theta), y=r\sin(\theta), x^2+y^2=r^2\)

Note that the last formula is a manipulation of a trignometric identity.

Simply replace these with x and y in the original function.

\(\displaystyle f(x)=2x^2+5x+2+2y^2\)

\(\displaystyle f(x)=(x^2+y^2)2+5x+2\)

\(\displaystyle f(x)=2r^2+5rcos(\theta)+2\)

Example Question #12 : Functions, Graphs, And Limits

What is the equation \(\displaystyle y=-\frac{1}{2}x^{2}\) in polar form?

Possible Answers:

\(\displaystyle r=\frac{1}{2}\tan \theta\sec\theta\)

\(\displaystyle r=-2\tan \theta\sec\theta\)

\(\displaystyle r=-\tan 2\theta\sec\theta\)

\(\displaystyle r=2\tan \theta\sec\theta\)

\(\displaystyle r=-\frac{1}{2}\tan \theta\sec\theta\)

Correct answer:

\(\displaystyle r=-2\tan \theta\sec\theta\)

Explanation:

We can convert from rectangular to polar form by using the following trigonometric identities: \(\displaystyle y=r\sin\theta\) and \(\displaystyle x=r\cos\theta\). Given \(\displaystyle y=-\frac{1}{2}x^{2}\), then:

\(\displaystyle r\sin\theta=-\frac{1}{2}(r\cos \theta)^{2}\)

\(\displaystyle r\sin\theta=-\frac{1}{2}r^{2}\cos^{2} \theta\)

 Dividing both sides by \(\displaystyle r\cos\theta\), we get:

\(\displaystyle \tan \theta=-\frac{1}{2}r\cos\theta\)

\(\displaystyle \frac{\tan \theta}{\cos\theta}=-\frac{1}{2}r\)

\(\displaystyle \tan \theta\sec\theta=-\frac{1}{2}r\)

\(\displaystyle r=-2\tan \theta\sec\theta\)

Example Question #3 : Polar Form

What is the polar form of \(\displaystyle y=-7x^{2}\)?

Possible Answers:

\(\displaystyle r=-\frac{1}{7}\tan\theta\sec\theta\)

\(\displaystyle r=\tan7\theta\sec\theta\)

\(\displaystyle r=7\tan\theta\sec\theta\)

\(\displaystyle r=\frac{1}{7}\tan\theta\sec\theta\)

\(\displaystyle r=-7\tan\theta\sec\theta\)

Correct answer:

\(\displaystyle r=-\frac{1}{7}\tan\theta\sec\theta\)

Explanation:

We can convert from rectangular to polar form by using the following trigonometric identities: and . Given \(\displaystyle y=-7x^{2}\), then:

\(\displaystyle r\sin\theta=-7(r\cos\theta)^{2}\)

\(\displaystyle r\sin\theta=-7r^{2}\cos^{2}\theta\)

Dividing both sides by , we get:

\(\displaystyle \tan\theta=-7r\cos\theta\)

\(\displaystyle \frac{\tan\theta}{\cos\theta}=-7r\)

\(\displaystyle \tan\theta\sec\theta=-7r\)

\(\displaystyle r=-\frac{1}{7}\tan\theta\sec\theta\)

Example Question #11 : Functions, Graphs, And Limits

What is the polar form of \(\displaystyle y=3-4x\)?

Possible Answers:

\(\displaystyle r=\frac{3}{\sin\theta+4\cos\theta}\)

\(\displaystyle r=-\frac{3}{\sin\theta-4\cos\theta}\)

\(\displaystyle r=\frac{3}{\sin\theta-4\cos\theta}\)

\(\displaystyle r=-\frac{3}{\sin\theta+4\cos\theta}\)

None of the above

Correct answer:

\(\displaystyle r=\frac{3}{\sin\theta+4\cos\theta}\)

Explanation:

We can convert from rectangular to polar form by using the following trigonometric identities: and . Given \(\displaystyle y=3-4x\), then:

\(\displaystyle r\sin\theta=3-4(r\cos\theta)\)

\(\displaystyle r\sin\theta+4(r\cos\theta)=3\)

\(\displaystyle r(\sin\theta+4\cos\theta)=3\)

\(\displaystyle r=\frac{3}{\sin\theta+4\cos\theta}\)

 

 

Example Question #14 : Functions, Graphs, And Limits

What is the polar form of \(\displaystyle y=-x^{2}\)?

Possible Answers:

\(\displaystyle r=\frac{\tan\theta}{\sec\theta}\)

None of the above

\(\displaystyle r=-\tan\theta\sec\theta\)

\(\displaystyle r=-\frac{\tan\theta}{\sec\theta}\)

\(\displaystyle r=\tan\theta\sec\theta\)

Correct answer:

\(\displaystyle r=-\tan\theta\sec\theta\)

Explanation:

We can convert from rectangular to polar form by using the following trigonometric identities: and . Given \(\displaystyle y=-x^{2}\), then:

\(\displaystyle r\sin\theta=-(r\cos\theta)^{2}\)

Dividing both sides by , we get:

\(\displaystyle \tan\theta=-r\cos\theta\)

\(\displaystyle \frac{\tan\theta}{\cos\theta}=-r\)

\(\displaystyle \tan\theta\sec\theta=-r\)

\(\displaystyle r=-\tan\theta\sec\theta\)

Example Question #1 : Polar Form

What is the polar form of \(\displaystyle y=-10x+5\)?

Possible Answers:

\(\displaystyle r=\frac{5}{\sin\theta-10\cos\theta}\)

\(\displaystyle r=-\frac{5}{\sin\theta+10\cos\theta}\)

\(\displaystyle r=-\frac{5}{\sin\theta-10\cos\theta}\)

\(\displaystyle r=\frac{5}{\sin\theta+10\cos\theta}\)

None of the above

Correct answer:

\(\displaystyle r=\frac{5}{\sin\theta+10\cos\theta}\)

Explanation:

We can convert from rectangular to polar form by using the following trigonometric identities: and . Given \(\displaystyle y=-10x+5\), then:

\(\displaystyle r\sin\theta=-10(r\cos\theta)+5\)

\(\displaystyle r\sin\theta+10(r\cos\theta)=5\)

\(\displaystyle r(\sin\theta+10\cos\theta)=5\)

\(\displaystyle r=\frac{5}{\sin\theta+10\cos\theta}\)

Learning Tools by Varsity Tutors