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D( x )
(15-x)^2 = 0
(15-x)^2 = 0
(15-x)^2 = 0
15-x = 0 // - 15
-x = -15 // * -1
x = 15
x in (-oo:15) U (15:+oo)
(27*x^4)/((15-x)^2) = 0
27*x^4 = 0 // : 27
x^4 = 0
x = 0
x = 0
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