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 . The area of the region that Little Albert colored in is equal to a π + c b 3 , where a , b , and c are positive integers, and b and c are coprime.
Find a + b + c .
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@Steven Yuan In polar coordinates we define 'r' as the distance from origin, and θ as the angle with positive x. Then this means that 'r' must be positive. But 3 − 6 sin θ can be negative. Where am I wrong?
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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)
But if we substitute theta= 3*pi/2 we get r=9 and not -9. Is it me who is wrong?
Notice in the graph, there is a loop inside of the shaded region where r is negative, or when sin ( θ ) > 2 1 . Therefore, we exclude all values in the interval ( 6 π , 6 5 π ) 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, 2 1 ⋅ b a s e ⋅ h e i g h t , we can obtain the series ∑ n = 0 ∞ 2 r n + r n + 1 ≈ ∫ a b 2 ( 3 − 6 sin ( θ ) ) 2 d θ . Now we solve for the integral ∫ a b 2 ( 3 − 6 sin ( θ ) ) 2 d θ ∫ a b 2 9 ( 1 − 2 sin ( θ ) ) 2 d θ 2 9 ⋅ ∫ a b 4 sin 2 ( θ ) − 4 sin ( θ ) + 1 d θ 2 9 ( 4 ∫ a b sin 2 ( θ ) d θ − 4 ∫ a b sin ( θ ) d θ + ∫ a b 1 d θ 2 9 ( 2 θ − 2 cos ( θ ) sin ( θ ) + 4 cos ( θ ) + θ ) 2 2 7 θ − 9 cos ( θ ) sin ( θ ) + 1 8 cos ( θ ) 2 2 7 θ − 9 cos ( θ ) ( sin ( θ ) − 2 ) Plugging this into our equation, we get [ 2 2 7 θ − 9 cos ( θ ) ( sin ( θ ) − 2 ) ] 0 6 π + [ 2 2 7 θ − 9 cos ( θ ) ( sin ( θ ) − 2 ) ] 6 5 π 2 π 2 7 π + 4 9 π − 4 4 5 π + 1 8 − 1 8 + 2 ( 9 4 3 ⋅ 3 ) 1 8 π + 2 2 7 3 Therefore, a = 1 8 , b = 2 7 , c = 2 , a + b + c = 1 8 + 2 7 + 2 = 4 7
Or you could just integrate from 5 pi/6 to 13 pi/6. That would simplify the evaluation of the integral at its bounds.
Area A = 2 9 ( ∫ 0 2 π ( 1 − 2 s i n θ ) d θ − ∫ 6 π 6 5 π ( 1 − 2 s i n θ ) d θ ) = 2 9 ( 6 π − ( 2 π − 3 3 ) ) = 2 9 ( 4 π + 3 3 ) =
1 8 π + 2 2 7 3 = a π + 2 b 3 ⟹ a + b + c = 4 7 .
Everyone is saying that b = 27, but 27 is not prime.
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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.
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First, we must determine for which values of θ does the limaçon cross the origin. To do that, we set r = 0 and solve:
3 − 6 sin θ sin θ θ = 0 = 2 1 = 6 π , 6 5 π .
Now, we focus on one part of the shaded area:
The region between θ = 6 5 π and 2 3 π
This region starts at θ = 6 5 π and ends at θ = 2 3 π . 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 ) between θ = α and β is 2 1 ∫ α β r ( θ ) 2 d θ . Let A be the area of the region Little Albert colored in. We have
A = 2 ⋅ 2 1 ∫ 6 5 π 2 3 π ( 3 − 6 sin θ ) 2 d θ = 9 ∫ 6 5 π 2 3 π ( 1 − 4 sin θ + 4 sin 2 θ ) d θ = 9 ∫ 6 5 π 2 3 π [ 1 − 4 sin θ + 4 ( 2 1 − cos 2 θ ) ] d θ = 9 ∫ 6 5 π 2 3 π ( 3 − 4 sin θ − 2 cos 2 θ ) d θ = 9 [ 3 θ + 4 cos θ − sin 2 θ ] 6 5 π 2 3 π = 9 [ ( 2 9 π + 4 cos 2 3 π − sin 3 π ) − ( 2 5 π + 4 cos 6 5 π − sin 3 5 π ) ] = 1 8 π + 2 2 7 3 .
Thus, a = 1 8 , b = 2 7 , and c = 2 , so a + b + c = 4 7 .