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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
1/21 = 28/x // - 28/x
1/21-(28/x) = 0
1/21-28*x^-1 = 0
-28*x^-1 = -1/21 // : -28
x^-1 = 1/588
-1 < 0
1/(x^1) = 1/588 // * x^1
1 = 1/588*x^1 // : 1/588
588 = x^1
x = 588
x = 588
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