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D( x )
(x-8)^2 = 0
(x-8)^2 = 0
(x-8)^2 = 0
x-8 = 0 // + 8
x = 8
x in (-oo:8) U (8:+oo)
4-(400/((x-8)^2)) = 0
4-400*(x-8)^-2 = 0
4-400/((x-8)^2) = 0
(4*(x-8)^2)/((x-8)^2)-400/((x-8)^2) = 0
4*(x-8)^2-400 = 0
4*x^2-64*x-144 = 0
4*x^2-64*x-144 = 0
4*(x^2-16*x-36) = 0
x^2-16*x-36 = 0
DELTA = (-16)^2-(-36*1*4)
DELTA = 400
DELTA > 0
x = (400^(1/2)+16)/(1*2) or x = (16-400^(1/2))/(1*2)
x = 18 or x = -2
4*(x+2)*(x-18) = 0
(4*(x+2)*(x-18))/((x-8)^2) = 0
(4*(x+2)*(x-18))/((x-8)^2) = 0 // * (x-8)^2
4*(x+2)*(x-18) = 0
( 4 )
4 = 0
x belongs to the empty set
( x+2 )
x+2 = 0 // - 2
x = -2
( x-18 )
x-18 = 0 // + 18
x = 18
x in { -2, 18 }
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