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D( q )
4-q = 0
4-q = 0
4-q = 0
4-q = 0 // - 4
-q = -4 // * -1
q = 4
q in (-oo:4) U (4:+oo)
4 = 8/(4-q) // - 8/(4-q)
4-(8/(4-q)) = 0
4-8*(4-q)^-1 = 0
4-8/(4-q) = 0
(4*(4-q))/(4-q)-8/(4-q) = 0
4*(4-q)-8 = 0
8-4*q = 0
(8-4*q)/(4-q) = 0
(8-4*q)/(4-q) = 0 // * 4-q
8-4*q = 0
8-4*q = 0 // - 8
-4*q = -8 // : -4
q = -8/(-4)
q = 2
q = 2
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