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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
400/5 = 11200/x // - 11200/x
400/5-(11200/x) = 0
400/5-11200*x^-1 = 0
-11200*x^-1 = -80 // : -11200
x^-1 = 1/140
-1 < 0
1/(x^1) = 1/140 // * x^1
1 = 1/140*x^1 // : 1/140
140 = x^1
x = 140
x = 140
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