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D( q )
q = 0
q = 0
q = 0
q in (-oo:0) U (0:+oo)
120/q = 40 // - 40
120/q-40 = 0
120*q^-1 = 40 // : 120
q^-1 = 1/3
-1 < 0
1/(q^1) = 1/3 // * q^1
1 = 1/3*q^1 // : 1/3
3 = q^1
q = 3
q = 3
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