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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6/21 = 8/x // - 8/x
6/21-(8/x) = 0
6/21-8*x^-1 = 0
-8*x^-1 = -2/7 // : -8
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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