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k in (-oo:+oo)
(1/2)*k-k+8/9 = 5/9 // - 5/9
(1/2)*k-k+8/9-(5/9) = 0
(1/2)*k-k+8/9-5/9 = 0
1/3-1/2*k = 0 // - 1/3
-1/2*k = -1/3 // : -1/2
k = -1/3/(-1/2)
k = 2/3
k = 2/3
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