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Calculus Level 3

For Mother's Day, Little Albert wants to show just how much he loves his mom by making a card for her with a big, colorful heart on the front. He gets out his paper, uncaps a marker, and gets to work.

The curve that Little Albert drew is called a limaçon . (The extra loop inside the heart will be invisible to his mom after coloring.) The polar equation for the particular limaçon that he drew is r = 3 6 sin θ . r = 3 - 6 \sin \theta. The area of the region that Little Albert colored in is equal to a π + b 3 c , a \pi + \frac{b \sqrt{3}}{c}, where a , a, b , b, and c c are positive integers, and b b and c c are coprime.

Find a + b + c . a + b + c.


The answer is 47.

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3 solutions

Steven Yuan
May 14, 2017

First, we must determine for which values of θ \theta does the limaçon cross the origin. To do that, we set r = 0 r = 0 and solve:

3 6 sin θ = 0 sin θ = 1 2 θ = π 6 , 5 π 6 . \begin{aligned} 3 - 6 \sin \theta &= 0 \\ \sin \theta &= \dfrac{1}{2} \\ \theta &= \dfrac{\pi}{6}, \dfrac{5\pi}{6}. \end{aligned}

Now, we focus on one part of the shaded area:

The region between \(\theta = \frac{5\pi}{6}\) and \(\frac{3\pi}{2}\) The region between θ = 5 π 6 \theta = \frac{5\pi}{6} and 3 π 2 \frac{3\pi}{2}

This region starts at θ = 5 π 6 \theta = \dfrac{5\pi}{6} and ends at θ = 3 π 2 \theta = \dfrac{3\pi}{2} . Notice that doubling this region gives the region that Little Albert colored in, so we want to find twice the area of that region.

Recall that the area of a polar curve represented by r ( θ ) r(\theta) between θ = α \theta = \alpha and β \beta is 1 2 α β r ( θ ) 2 d θ \dfrac{1}{2} \displaystyle \int_\alpha^\beta r(\theta)^2 \, \mathrm{d} \theta . Let A A be the area of the region Little Albert colored in. We have

A = 2 1 2 5 π 6 3 π 2 ( 3 6 sin θ ) 2 d θ = 9 5 π 6 3 π 2 ( 1 4 sin θ + 4 sin 2 θ ) d θ = 9 5 π 6 3 π 2 [ 1 4 sin θ + 4 ( 1 cos 2 θ 2 ) ] d θ = 9 5 π 6 3 π 2 ( 3 4 sin θ 2 cos 2 θ ) d θ = 9 [ 3 θ + 4 cos θ sin 2 θ ] 5 π 6 3 π 2 = 9 [ ( 9 π 2 + 4 cos 3 π 2 sin 3 π ) ( 5 π 2 + 4 cos 5 π 6 sin 5 π 3 ) ] = 18 π + 27 3 2 . \begin{aligned} A &= 2 \cdot \dfrac{1}{2} \displaystyle \int_\frac{5\pi}{6}^\frac{3\pi}{2} (3 - 6 \sin \theta)^2 \, \mathrm{d} \theta \\ &= 9 \displaystyle \int_\frac{5\pi}{6}^\frac{3\pi}{2} \left(1 - 4 \sin \theta + 4 \sin^2 \theta \right) \, \mathrm{d} \theta \\ &= 9 \displaystyle \int_\frac{5\pi}{6}^\frac{3\pi}{2} \left[1 - 4 \sin \theta + 4 \left(\dfrac{1 - \cos 2\theta}{2} \right)\right] \, \mathrm{d} \theta \\ &= 9 \displaystyle \int_\frac{5\pi}{6}^\frac{3\pi}{2} (3 - 4 \sin \theta - 2 \cos 2\theta) \, \mathrm{d} \theta \\ &= 9 \left [3 \theta + 4 \cos \theta - \sin 2 \theta \right]_\frac{5\pi}{6}^\frac{3\pi}{2} \\ &= 9 \left [ \left (\dfrac{9\pi}{2} + 4 \cos \dfrac{3\pi}{2} - \sin 3\pi \right) - \left (\dfrac{5\pi}{2} + 4 \cos \dfrac{5\pi}{6} - \sin \dfrac{5\pi}{3} \right) \right] \\ &= 18\pi + \dfrac{27 \sqrt{3}}{2}. \end{aligned}

Thus, a = 18 a = 18 , b = 27 b = 27 , and c = 2 c = 2 , so a + b + c = 47 a + b + c = \boxed{47} .

@Steven Yuan In polar coordinates we define 'r' as the distance from origin, and θ \theta as the angle with positive x. Then this means that 'r' must be positive. But 3 6 sin θ 3-6\sin{\theta} can be negative. Where am I wrong?

Satvik Golechha - 4 years ago

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r is the distance from the origin along the angle theta. When 3-6sin(theta) is negative, we simply represent it as going backwards. The absolute value of r is still the distance, though.

For example, theta = 0 and r = -3 is the same as theta = 180 and r = 3 is the same as (-3,0)

Alex Li - 4 years ago

But if we substitute theta= 3*pi/2 we get r=9 and not -9. Is it me who is wrong?

Siddharth Singh - 4 years ago

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Yeah, I wrote it wrong in the diagram. Just ignore it.

Steven Yuan - 4 years ago
Kevin Tong
May 21, 2017

Notice in the graph, there is a loop inside of the shaded region where r is negative, or when sin ( θ ) > 1 2 \sin(\theta) > \frac{1}{2} . Therefore, we exclude all values in the interval ( π 6 , 5 π 6 ) (\frac{\pi}{6}, \frac{5\pi}{6}) since that region of the polar expression would've been redundant. Now, in order to find the area of the region bounded by the polar expression between 2 points, we can form infinitesimal triangles. Using the triangle formula, 1 2 b a s e h e i g h t \frac{1}{2} \cdot base \cdot height , we can obtain the series n = 0 r n + r n + 1 2 a b ( 3 6 sin ( θ ) ) 2 2 d θ \sum_{n=0}^{\infty} \frac{r_{n} + r_{n+1}}{2} \approx \int_{a}^{b} \frac{(3-6\sin(\theta))^2}{2} d\theta . Now we solve for the integral a b ( 3 6 sin ( θ ) ) 2 2 d θ a b 9 ( 1 2 sin ( θ ) ) 2 2 d θ 9 2 a b 4 sin 2 ( θ ) 4 sin ( θ ) + 1 d θ 9 2 ( 4 a b sin 2 ( θ ) d θ 4 a b sin ( θ ) d θ + a b 1 d θ 9 2 ( 2 θ 2 cos ( θ ) sin ( θ ) + 4 cos ( θ ) + θ ) 27 θ 2 9 cos ( θ ) sin ( θ ) + 18 cos ( θ ) 27 θ 2 9 cos ( θ ) ( sin ( θ ) 2 ) \int_{a}^{b} \frac{(3-6\sin(\theta))^2}{2} d\theta \\ \int_{a}^{b} \frac{9(1-2\sin(\theta))^2}{2} d\theta \\ \frac{9}{2} \cdot \int_{a}^{b} 4\sin^2(\theta) - 4\sin(\theta) +1 d\theta \\ \frac{9}{2}(4\int_{a}^{b}\sin^2(\theta)d\theta-4\int_{a}^{b}\sin(\theta)d\theta+\int_{a}^{b}1d\theta \\ \frac{9}{2}(2\theta-2\cos(\theta)\sin(\theta)+4\cos(\theta)+\theta) \\ \frac{27\theta}{2}-9\cos(\theta)\sin(\theta)+18\cos(\theta) \\ \frac{27\theta}{2}-9\cos(\theta)(\sin(\theta)-2) Plugging this into our equation, we get [ 27 θ 2 9 cos ( θ ) ( sin ( θ ) 2 ) ] 0 π 6 + [ 27 θ 2 9 cos ( θ ) ( sin ( θ ) 2 ) ] 5 π 6 2 π 27 π + 9 π 4 45 π 4 + 18 18 + 2 ( 9 3 3 4 ) 18 π + 27 3 2 [\frac{27\theta}{2}-9\cos(\theta)(\sin(\theta)-2)]_{0}^{\frac{\pi}{6}} + [\frac{27\theta}{2}-9\cos(\theta)(\sin(\theta)-2)]_{\frac{5\pi}{6}}^{2\pi} \\ 27\pi+\frac{9\pi}{4}-\frac{45\pi}{4}+18-18+2(9\frac{\sqrt{3} \cdot 3}{4}) \\ 18\pi+\frac{27\sqrt{3}}{2} Therefore, a = 18 , b = 27 , c = 2 , a + b + c = 18 + 27 + 2 = 47 a=18, b=27, c=2, a+b+c = 18+27+2=\boxed{47}

Or you could just integrate from 5 pi/6 to 13 pi/6. That would simplify the evaluation of the integral at its bounds.

Richard Desper - 4 years ago
Rocco Dalto
May 24, 2017

Area A = 9 2 ( 0 2 π ( 1 2 s i n θ ) d θ π 6 5 π 6 ( 1 2 s i n θ ) d θ ) = 9 2 ( 6 π ( 2 π 3 3 ) ) = 9 2 ( 4 π + 3 3 ) = A = \dfrac{9}{2} (\int_{0}^{2\pi} (1 - 2 sin\theta) d\theta - \int_{\dfrac{\pi}{6}}^{\dfrac{5\pi}{6}} (1 - 2 sin\theta) d\theta) = \dfrac{9}{2} (6\pi - (2\pi - 3 \sqrt{3})) = \dfrac{9}{2} (4\pi + 3 \sqrt{3}) =

18 π + 27 3 2 = a π + b 3 2 a + b + c = 47. 18 \pi + \dfrac{27 \sqrt{3}}{2} = a\pi + \dfrac{b\sqrt{3}}{2} \implies a + b + c = 47.

Everyone is saying that b = 27, but 27 is not prime.

John Tyner - 4 years ago

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This may have been edited since you made this comment, but the way the problem reads now is that b and c must be coprime, not necessarily prime.

Daniel Juncos - 4 years ago

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