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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
9/21 = 3/x // - 3/x
9/21-(3/x) = 0
9/21-3*x^-1 = 0
-3*x^-1 = -3/7 // : -3
x^-1 = 1/7
-1 < 0
1/(x^1) = 1/7 // * x^1
1 = 1/7*x^1 // : 1/7
7 = x^1
x = 7
x = 7
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