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D( q )
q = 0
q = 0
q = 0
q in (-oo:0) U (0:+oo)
4/q = 6/5 // - 6/5
4/q-(6/5) = 0
4/q-6/5 = 0
4*q^-1 = 6/5 // : 4
q^-1 = 3/10
-1 < 0
1/(q^1) = 3/10 // * q^1
1 = 3/10*q^1 // : 3/10
10/3 = q^1
q = 10/3
q = 10/3
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