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D( x )
x = 0
x-4 = 0
x = 0
x = 0
x-4 = 0
x-4 = 0
x-4 = 0 // + 4
x = 4
x in (-oo:0) U (0:4) U (4:+oo)
585/x = 585/(x-4)-20 // - 585/(x-4)-20
585/x-(585/(x-4))+20 = 0
585/x-585*(x-4)^-1+20 = 0
585/x-585/(x-4)+20 = 0
(-585*x)/(x*(x-4))+(585*(x-4))/(x*(x-4))+(20*x*(x-4))/(x*(x-4)) = 0
585*(x-4)+20*x*(x-4)-585*x = 0
20*x^2-80*x-2340 = 0
20*x^2-80*x-2340 = 0
20*(x^2-4*x-117) = 0
x^2-4*x-117 = 0
DELTA = (-4)^2-(-117*1*4)
DELTA = 484
DELTA > 0
x = (484^(1/2)+4)/(1*2) or x = (4-484^(1/2))/(1*2)
x = 13 or x = -9
20*(x+9)*(x-13) = 0
(20*(x+9)*(x-13))/(x*(x-4)) = 0
(20*(x+9)*(x-13))/(x*(x-4)) = 0 // * x*(x-4)
20*(x+9)*(x-13) = 0
( 20 )
20 = 0
x belongs to the empty set
( x+9 )
x+9 = 0 // - 9
x = -9
( x-13 )
x-13 = 0 // + 13
x = 13
x in { -9, 13 }
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