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D( x )
x+4 = 0
x = 0
x+4 = 0
x+4 = 0
x+4 = 0 // - 4
x = -4
x = 0
x = 0
x in (-oo:-4) U (-4:0) U (0:+oo)
480/x-(480/(x+4)) = 6 // - 6
480/x-(480/(x+4))-6 = 0
480/x-480*(x+4)^-1-6 = 0
480/x-480/(x+4)-6 = 0
(-480*x)/(x*(x+4))+(480*(x+4))/(x*(x+4))+(-6*x*(x+4))/(x*(x+4)) = 0
480*(x+4)-6*x*(x+4)-480*x = 0
1920-6*x^2-24*x = 0
1920-6*x^2-24*x = 0
6*(320-x^2-4*x) = 0
320-x^2-4*x = 0
DELTA = (-4)^2-(-1*4*320)
DELTA = 1296
DELTA > 0
x = (1296^(1/2)+4)/(-1*2) or x = (4-1296^(1/2))/(-1*2)
x = -20 or x = 16
6*(x+20)*(x-16) = 0
(6*(x+20)*(x-16))/(x*(x+4)) = 0
(6*(x+20)*(x-16))/(x*(x+4)) = 0 // * x*(x+4)
6*(x+20)*(x-16) = 0
( x+20 )
x+20 = 0 // - 20
x = -20
( x-16 )
x-16 = 0 // + 16
x = 16
x in { -20, 16 }
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