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k in (-oo:+oo)
4*(4-1*k) = 20 // - 20
4*(4-1*k)-20 = 0
4*(4-k)-20 = 0
4*(4-k)-20 = 0
-4*k-4 = 0
-4*k-4 = 0
-4*k-4 = 0 // + 4
-4*k = 4 // : -4
k = 4/(-4)
k = -1
k = -1
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