L
letuananh1991


cho p q a b c d tuỳ ý thoả mãn
p^2+q^2-a^2-b^2-c^2-d^2>0
CM
(p^2-a^2-b^2)(q^2-c^2-d^2)<hoac= (pq-ac-bd)^2
p^2+q^2-a^2-b^2-c^2-d^2>0
CM
(p^2-a^2-b^2)(q^2-c^2-d^2)<hoac= (pq-ac-bd)^2