Câu hỏi 180
Cho $a$, $b$, $c$, $d$ là các số thực. Chứng minh rằng:
$$
\left(a^{2}-a+1\right)\left(b^{2}-b+1\right)\left(c^{2}-c+1\right)\left(d^{2}-d+1\right) \geq \frac{9}{16}(a-b)(b-c)(c-d)(d-a)
$$
$$
\left(a^{2}-a+1\right)\left(b^{2}-b+1\right)\left(c^{2}-c+1\right)\left(d^{2}-d+1\right) \geq \frac{9}{16}(a-b)(b-c)(c-d)(d-a)
$$