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