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D( x )
x = 0
x-3 = 0
x = 0
x = 0
x-3 = 0
x-3 = 0
x-3 = 0 // + 3
x = 3
x in (-oo:0) U (0:3) U (3:+oo)
84/(x-3) = 84/x+9 // - 84/x+9
84/(x-3)-(84/x)-9 = 0
84/(x-3)-84*x^-1-9 = 0
84/(x-3)-84/x-9 = 0
(84*x)/(x*(x-3))+(-84*(x-3))/(x*(x-3))+(-9*x*(x-3))/(x*(x-3)) = 0
84*x-84*(x-3)-9*x*(x-3) = 0
27*x-9*x^2+252 = 0
27*x-9*x^2+252 = 0
9*(3*x-x^2+28) = 0
3*x-x^2+28 = 0
DELTA = 3^2-(-1*4*28)
DELTA = 121
DELTA > 0
x = (121^(1/2)-3)/(-1*2) or x = (-121^(1/2)-3)/(-1*2)
x = -4 or x = 7
9*(x+4)*(x-7) = 0
(9*(x+4)*(x-7))/(x*(x-3)) = 0
(9*(x+4)*(x-7))/(x*(x-3)) = 0 // * x*(x-3)
9*(x+4)*(x-7) = 0
( x+4 )
x+4 = 0 // - 4
x = -4
( x-7 )
x-7 = 0 // + 7
x = 7
x in { -4, 7 }
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