y = kx+b与x轴交于点B(1, 0): k + b = 0, b = -k
要使不等式组-2b 0, k < 0
不等式变为: 2k < kx - k ≤0
都加k: 3k < kx ≤ k
都除以k: 3 > x ≥ 1