a, – x^2 + 4x – 5 < 0 <=> – x^2 + 4x – 4 – 1 < 0 <=> – (x^2 – 4x + 4) – 1 < 0 <=> – (x – 2)^2 – 1 < 0 Ta có: (x – 2)^2 \ge 0 <=> – (x – 2)^2 \ge 0 Do đó: – (x – 2)^2 – 1 < 0 luôn đúng AA x in RR b, x^2 – x + 1 > 0 <=> x^2 – x + 1/4 + 3/4 > 0 <=> (x – 1/2)^2 + 3/4 > 0 Ta có: (x – 1/2)^2 \ge 0 <=> (x – 1/2)^2 + 3/4 \ge 3/4 Do đó: (x – 1/2)^2 + 3/4 > 0 luôn đúng AA x in RR Trả lời
2 bình luận về “a, -x^2+4x-5<0 b, x^2-x+1>0”