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D( q )
q = 0
q = 0
q = 0
q in (-oo:0) U (0:+oo)
3 = 6/q // - 6/q
3-(6/q) = 0
3-6*q^-1 = 0
-6*q^-1 = -3 // : -6
q^-1 = 1/2
-1 < 0
1/(q^1) = 1/2 // * q^1
1 = 1/2*q^1 // : 1/2
2 = q^1
q = 2
q = 2
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