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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
20/7 = 80/x // - 80/x
20/7-(80/x) = 0
20/7-80*x^-1 = 0
-80*x^-1 = -20/7 // : -80
x^-1 = 1/28
-1 < 0
1/(x^1) = 1/28 // * x^1
1 = 1/28*x^1 // : 1/28
28 = x^1
x = 28
x = 28
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