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k in (-oo:+oo)
21/15 = 14*k // - 14*k
21/15-(14*k) = 0
21/15-14*k = 0
7/5-14*k = 0 // - 7/5
-14*k = -7/5 // : -14
k = -7/5/(-14)
k = 1/10
k = 1/10
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