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k in (-oo:+oo)
225/210 = 10*k // - 10*k
225/210-(10*k) = 0
225/210-10*k = 0
15/14-10*k = 0 // - 15/14
-10*k = -15/14 // : -10
k = -15/14/(-10)
k = 3/28
k = 3/28
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