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Z in (-oo:+oo)
6/12 = Z/21 // - Z/21
6/12-(Z/21) = 0
1/2-1/21*Z = 0 // - 1/2
-1/21*Z = -1/2 // : -1/21
Z = -1/2/(-1/21)
Z = 21/2
Z = 21/2
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