Pyramids and Circles!

Level pending

In A B C D \square{ABCD} , the circle with center O O passes through vertices A A and D D and is tangent to B C BC at E E .

Folding the arc of the semi-circle at a right angle, as shown above, the radius of the semicircle becomes the height of the pyramid.

Let A A be the lateral surface area of the pyramid above.

If A A A B C D = a + b c + c d e \dfrac{A}{A_{\square{ABCD}}} = \dfrac{\sqrt{a} + b\sqrt{c} + c\sqrt{d}}{e} , where a , b , c , d a,b,c,d and e e are coprime positive integers, find

a + b + c + d + e a + b + c + d + e .


The answer is 86.

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1 solution

Rocco Dalto
Apr 28, 2020

Let 2 z 2z be the length of a side of square base A B C D ABCD and let A : ( 0 , 0 ) , B : ( 0 , 2 z ) , A:(0,0), B:(0,2z),

C : ( 2 z , 2 z ) , D : ( 2 z , 0 ) C:(2z,2z), D:(2z,0) and E : ( z , 2 z ) E:(z,2z) and O : ( x , y ) O:(x,y) .

( 1 ) : ( x z ) 2 + ( y 2 z ) 2 = r 2 (1): (x - z)^2 + (y - 2z)^2 = r^2

( 2 ) : x 2 + y 2 = r 2 (2): x^2 + y^2 = r^2

( 3 ) : ( x 2 z ) 2 + y 2 = r 2 (3): (x - 2z)^2 + y^2 = r^2

Subtracting ( 2 ) (2) from ( 1 ) 2 z x + 4 z y = 5 z 2 (1) \implies 2zx + 4zy = 5z^2

and

Subtracting ( 2 ) (2) from ( 3 ) 4 z ( z x ) = 0 (3) \implies 4z(z - x) = 0 and z 0 x = z z \neq 0 \implies x =z \implies

y = 3 4 z r = 5 4 z y = \dfrac{3}{4}z \implies r = \dfrac{5}{4}z

For triangular face A P D APD the slant height s 1 = ( 5 z 4 ) 2 + ( 3 z 4 ) 2 = s_{1} = \sqrt{(\dfrac{5z}{4})^2 + (\dfrac{3z}{4})^2} = 34 4 z \dfrac{\sqrt{34}}{4}z

For triangular face B P C BPC the slant height s 2 = ( 5 z 4 ) 2 + ( 5 z 4 ) 2 = s_{2} = \sqrt{(\dfrac{5z}{4})^2 + (\dfrac{5z}{4})^2} = 5 2 4 z \dfrac{5\sqrt{2}}{4}z

For triangular faces A P B APB and D P C DPC the slant heights s 3 = s 4 = ( 5 z 4 ) 2 + z 2 = s_{3} = s_{4} = \sqrt{(\dfrac{5z}{4})^2 + z^2} = 41 4 z \dfrac{\sqrt{41}}{4}z

\implies The lateral surface area A = 34 + 5 2 + 2 41 4 z 2 A = \dfrac{\sqrt{34} + 5\sqrt{2} + 2\sqrt{41}}{4}z^2

A A A B C D = 34 + 5 2 + 2 41 4 = \implies \dfrac{A}{A_{\square{ABCD}}} = \dfrac{\sqrt{34} + 5\sqrt{2} + 2\sqrt{41}}{4} =

a + b c + c d e a + b + c + d + e = 86 \dfrac{\sqrt{a} + b\sqrt{c} + c\sqrt{d}}{e} \implies a + b + c + d + e = \boxed{86} .

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