In , the circle with center passes through vertices and and is tangent to at .
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 be the lateral surface area of the pyramid above.
If , where and are coprime positive integers, find
.
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Let 2 z be the length of a side of square base A B C D and let A : ( 0 , 0 ) , B : ( 0 , 2 z ) ,
C : ( 2 z , 2 z ) , D : ( 2 z , 0 ) and E : ( z , 2 z ) and O : ( x , y ) .
( 1 ) : ( x − z ) 2 + ( y − 2 z ) 2 = r 2
( 2 ) : x 2 + y 2 = r 2
( 3 ) : ( x − 2 z ) 2 + y 2 = r 2
Subtracting ( 2 ) from ( 1 ) ⟹ 2 z x + 4 z y = 5 z 2
and
Subtracting ( 2 ) from ( 3 ) ⟹ 4 z ( z − x ) = 0 and z = 0 ⟹ x = z ⟹
y = 4 3 z ⟹ r = 4 5 z
For triangular face A P D the slant height s 1 = ( 4 5 z ) 2 + ( 4 3 z ) 2 = 4 3 4 z
For triangular face B P C the slant height s 2 = ( 4 5 z ) 2 + ( 4 5 z ) 2 = 4 5 2 z
For triangular faces A P B and D P C the slant heights s 3 = s 4 = ( 4 5 z ) 2 + z 2 = 4 4 1 z
⟹ The lateral surface area A = 4 3 4 + 5 2 + 2 4 1 z 2
⟹ A □ A B C D A = 4 3 4 + 5 2 + 2 4 1 =
e a + b c + c d ⟹ a + b + c + d + e = 8 6 .