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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
25/40 = 10/x // - 10/x
25/40-(10/x) = 0
25/40-10*x^-1 = 0
-10*x^-1 = -5/8 // : -10
x^-1 = 1/16
-1 < 0
1/(x^1) = 1/16 // * x^1
1 = 1/16*x^1 // : 1/16
16 = x^1
x = 16
x = 16
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