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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(y+5)/x = -4/5 // + -4/5
(y+5)/x-(-4/5) = 0
(y+5)/x+4/5 = 0
(5*(y+5))/(5*x)+(4*x)/(5*x) = 0
5*(y+5)+4*x = 0
4*x+5*y+25 = 0
(4*x+5*y+25)/(5*x) = 0
(4*x+5*y+25)/(5*x) = 0 // * 5*x
4*x+5*y+25 = 0
4*x+5*y+25 = 0 // - 5*y+25
4*x = -(5*y+25) // : 4
x = (-(5*y+25))/4
x = (-(5*y+25))/4
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