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D( q )
q = 0
q = 0
q = 0
q in (-oo:0) U (0:+oo)
646/q = -19 // + 19
646/q+19 = 0
646*q^-1 = -19 // : 646
q^-1 = -1/34
-1 < 0
1/(q^1) = -1/34 // * q^1
1 = -1/34*q^1 // : -1/34
-34 = q^1
q = -34
q = -34
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