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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.2*y = 4/x-s // - 4/x-s
s-(4/x)+0.2*y = 0
s-4*x^-1+0.2*y = 0
-4*x^-1 = -(s+0.2*y) // : -4
x^-1 = (s+0.2*y)/4
-1 < 0
1/(x^1) = (s+0.2*y)/4 // * x^1
1 = x^1*((s+0.2*y)/4) // : (s+0.2*y)/4
4*(s+0.2*y)^-1 = x^1
x = 4*(s+0.2*y)^-1
x = 4*(s+0.2*y)^-1
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