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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
b^3*x^-2*((84*x^4*y^-3)/14) = 0
6*b^3*x^2*y^-3 = 0 // : 6*b^3*y^-3
x^2 = 0
x = 0
x in { 0}
x belongs to the empty set
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