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r in (-oo:+oo)
664/996 = r/12 // - r/12
664/996-(r/12) = 0
2/3-1/12*r = 0 // - 2/3
-1/12*r = -2/3 // : -1/12
r = -2/3/(-1/12)
r = 8
r = 8
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