What is the area?

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2 Answers
Dec 9, 2017

There is no real solution to this question.

Explanation:

The integral is not valid, if you look at what happens when theta approaches 0, you will see the function goes large, thus the area between [0,1] is kind of infinity.
---by @miles-a

I'm sorry. I don't know the steps of solving this. But you can try the formula mentioned by @alvin-l-2 as the following.

Dec 9, 2017

The calculation is hard. I used http://www.wolframalpha.com/ and obtained the result(about #0.50754#).

Explanation:

This is the graph of #r=1/(1-cos3theta)# (by Wolframalpha).
http://www.wolframalpha.com/input/?i=polar+r%3D1%2F(1-cos(3theta))

Then, how to interpret the graph?
[1] The graph crosses #x=0# at #theta=pi/2#. There,
#r=1/(1-cos((3pi)/2))=1#.

[2] When does the graph cross #x=1#?
To solve this, apply the formula #x=rcostheta#.
#1=rcostheta#
#r=1/costheta#
#1/(1-cos3theta)=1/costheta#
#1-cos3theta=costheta#
#cos3theta+costheta=1#

Use the formula #cos3theta=4cos^3theta-3costheta#.
#4cos^3theta-2costheta-1=0#

The real solution for this is:
#costheta≒0.88465#
#theta≒0.48505# [rad] #≒27.79# [deg]

In cartesian form, the graph crosses #x=0# at #(0,1)# and crosses #x=1# at #(1,tan(0.48505))=(1,0.52705)#.

Now we are ready to calculate the area.
The area bounded by polar curves is #S=1/2int_alpha^beta{f(theta)}^2d##theta#.
In this problem,
#S=1/2int_0.48505^(pi/2)(1/(1-cos3theta))^2d##theta=0.24401#.

However, this integration is done in polar coordination. You must add the area of the right triangle with vertices(in cartesian form) #(0,0)#, #(1,0)# and #(1,0.52705)#.

The answer is #0.24401+1/2*1*0.52705=0.50754#.

By the way, calculating areas bounded by polar curves is so difficult. I don't think it is necessary for us to do these types of calculation without a calculator.

I posted a question https://socratic.org/questions/calculating-areas-bounded-by-polar-curves-looks-extremely-difficult-do-americans two months ago but nobody has answered this. What are your opinions?