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D( x )
(75*x^2*z^4)/21 = 0
(75*x^2*z^4)/21 = 0
(75*x^2*z^4)/21 = 0
25/7*x^2*z^4 = 0 // : 25/7*z^4
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
((25*x^2*z^2)/9)/((75*x^2*z^4)/21) = 0
7/9*z^-2 = 0
x należy do R
x in (-oo:0) U (0:+oo)
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