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Geometry Level 3

Determine the area bounded by the graph ( y + 2 ) 2 = 8 x x 2 (|y|+2)^2=8x-x^2 . If the answer is in the form a π b c b \dfrac{a\pi}b -c\sqrt b , determine the value of a + b + c a+b+c .

Note: A scientific calculator is allowed, but please refrain from using a graphing calculator.


The answer is 43.

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1 solution

Yashas Ravi
Oct 28, 2019

The equation can be rewritten as ( y + 2 ) 2 + ( x 4 ) 2 = 16 (|y|+2)^2+(x-4)^2=16 . If y |y| is replaced with y y , the equation would be a circle centered at ( 4 , 2 ) (4,-2) . Because of the absolute value, the region of the circle below the x x -axis is replaced with the region of the graph above the x x -axis. In other words, the part of the graph above the x x -axis is reflected over the x x -axis, which means the total area is twice the area bound by the part of the graph above the x x -axis and the line y = 0 y=0 .

We know that the radius is 4 4 so the graph never enters the 2nd or 3rd quadrant. This means the area above the x x -axis is a circular cap with the base being the part of the x x -axis contained within the circle. The distance of the chord that represents the part of the x x -axis contained in the circle can be found by setting y = 0 y=0 , meaning x = 4 + 2 3 x=4+2√3 and x = 4 2 3 x=4-2√3 so the distance of the chord would be 4 3 4√3 .

We can draw radii from ( 4 , 2 ) (4,-2) to the endpoints of this chord and find the angle between the radii using the Law of Cosines since we know the radii are both 4 4 . The angle comes out to be 120 ˙ 120˙ and the area of the circular cap would be 16 π 3 \frac{16π}{3} ( 0.5 4 2 0.5 sin 120 ) -(0.5*4^2*0.5*\sin{120}) and multiplying this by 2 2 accounts for the reflection across the x x -axis. The final area comes out to be 32 π 3 \frac{32π}{3} 8 3 -8√3 so ( a + b + c ) = 32 + 8 + 3 = 43 (a+b+c)=32+8+3=43 which is the final answer.

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