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D( x )
20-(8*x) = 0
8*x-12 = 0
20-(8*x) = 0
20-(8*x) = 0
20-8*x = 0
20-8*x = 0 // - 20
-8*x = -20 // : -8
x = -20/(-8)
x = 5/2
8*x-12 = 0
8*x-12 = 0
8*x-12 = 0 // + 12
8*x = 12 // : 8
x = 12/8
x = 3/2
x in (-oo:3/2) U (3/2:5/2) U (5/2:+oo)
(4*x)/(20-(8*x)) = (4*x)/(8*x-12) // - (4*x)/(8*x-12)
(4*x)/(20-(8*x))-((4*x)/(8*x-12)) = 0
(4*x)/(20-8*x)-4*x*(8*x-12)^-1 = 0
(4*x)/(20-8*x)+(-4*x)/(8*x-12) = 0
(4*x*(8*x-12))/((20-8*x)*(8*x-12))+(-4*x*(20-8*x))/((20-8*x)*(8*x-12)) = 0
4*x*(8*x-12)-4*x*(20-8*x) = 0
64*x^2-128*x = 0
64*x^2-128*x = 0
64*x*(x-2) = 0
x-2 = 0 // + 2
x = 2
64*x*(x-2) = 0
(64*x*(x-2))/((20-8*x)*(8*x-12)) = 0
(64*x*(x-2))/((20-8*x)*(8*x-12)) = 0 // * (20-8*x)*(8*x-12)
64*x*(x-2) = 0
( 64*x )
64*x = 0 // : 64
x = 0
( x-2 )
x-2 = 0 // + 2
x = 2
x in { 0, 2 }
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