先假设k大于4/5,再假设可以小于4/5，然后再根据条件，你慢慢求吧！
|(4/k-5)| * |(5k-4)| ≤ 10
|(4/k-5)(5k-4)| ≤ 10
| 20-25k-16/k+20| ≤ 10
| 25k + 16/k + 40 | ≤ 10
-10 ≤ 25k + 16/k + 40 ≤ 10
-50 ≤ 25k + 16/k ≤ 30
(25k²+50k+16)/k ≥ 0 并且(25k²-30k+16)/k ≤ 0
(5k+8)(5k+2)/k ≥ 0并且k＜0【∵25k²-30k+16恒大于0】
∴ (5k+8)(5k+2) ≤ 0
∴-8/5 ≤ k ≤ -2/5
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