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D( X )
X = 0
64/X+20 = 0
X = 0
X = 0
64/X+20 = 0
64/X+20 = 0
64*X^-1 = -20 // : 64
X^-1 = -5/16
-1 < 0
1/(X^1) = -5/16 // * X^1
1 = -5/16*X^1 // : -5/16
-16/5 = X^1
X = -16/5
X in (-oo:-16/5) U (-16/5:0) U (0:+oo)
20/(64/X+20)+5 = 0
20/(64*X^-1+20)+5 = 0
20/(64*X^-1+20)+(5*(64*X^-1+20))/(64*X^-1+20) = 0
5*(64*X^-1+20)+20 = 0
320*X^-1+120 = 0
(320*X^-1+120)/(64*X^-1+20) = 0
(320*X^-1+120)/(64*X^-1+20) = 0 // * 64*X^-1+20
320*X^-1+120 = 0
320*X^-1 = -120 // : 320
X^-1 = -3/8
-1 < 0
1/(X^1) = -3/8 // * X^1
1 = -3/8*X^1 // : -3/8
-8/3 = X^1
X = -8/3
X = -8/3
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