Ta có:
$2012=( abc+bcd+cda+dab-a-b-c-d)^2 =( (ab - 1)(c + d) + (cd - 1)( a + b))$
$\leq ((ab -1)^2 +(a + b)^2)((cd -1)^2 +(c + d)^2)$
$=(a^2b^2 +a^2 +b^2+1)(c^2d^2 +c^2 +d^2 +1) =(a^2 +1)(b^2 +1)(c^2 +1)(d^2 +1) \Rightarrow Q.E.D$
- hoangmanhquan yêu thích