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D( x )
25-x = 0
25-x = 0
25-x = 0
25-x = 0 // - 25
-x = -25 // * -1
x = 25
x in (-oo:25) U (25:+oo)
(480-x)/(25-x) = 0
480-x = 0 // - 480
-x = -480 // * -1
x = 480
x = 480
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