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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
9/x = 12/21 // - 12/21
9/x-(12/21) = 0
9/x-4/7 = 0
9*x^-1 = 4/7 // : 9
x^-1 = 4/63
-1 < 0
1/(x^1) = 4/63 // * x^1
1 = 4/63*x^1 // : 4/63
63/4 = x^1
x = 63/4
x = 63/4
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