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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(1000*(1-(x/500))^2)/x = 0
(1000*(1-1/500*x)^2)/x = 0
( 1-1/500*x )
1-1/500*x = 0 // - 1
-1/500*x = -1 // : -1/500
x = -1/(-1/500)
x = 500
( 1000 )
1000 = 0
x belongs to the empty set
x in { 500, 500 }
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