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D( q )
q = 0
q = 0
q = 0
q in (-oo:0) U (0:+oo)
4/5 = 16/q // - 16/q
4/5-(16/q) = 0
4/5-16*q^-1 = 0
-16*q^-1 = -4/5 // : -16
q^-1 = 1/20
-1 < 0
1/(q^1) = 1/20 // * q^1
1 = 1/20*q^1 // : 1/20
20 = q^1
q = 20
q = 20
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