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k in (-oo:+oo)
4/12 = k/15 // - k/15
4/12-(k/15) = 0
1/3-1/15*k = 0 // - 1/3
-1/15*k = -1/3 // : -1/15
k = -1/3/(-1/15)
k = 5
k = 5
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