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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
7/12 = 21/x // - 21/x
7/12-(21/x) = 0
7/12-21*x^-1 = 0
-21*x^-1 = -7/12 // : -21
x^-1 = 1/36
-1 < 0
1/(x^1) = 1/36 // * x^1
1 = 1/36*x^1 // : 1/36
36 = x^1
x = 36
x = 36
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