Đặt : $A = 1.3.5 + 3.5.7 + 5.7.9 + ...+ 95.97.99 $
$⇒8A = 1.3.5.8 + 3.5.7.8 + ....+ 95.97.99.8 $
$⇒ 8A = 1.3.5.(1+7) + 3.5.7.(9-1) + ...+ 95.97.99.(101-93) $
$⇒ 8A = 1.3.5 + 1.3.5.7 + 3.5.7.9 - 1.3.5.7 + ....+ 95.97.99.101-93.95.97.99$
$⇒ 8A = 1.3.5 + 95.97.99.101 $
$⇒ A = \frac{1.3.5+95.97.99.101}{8} = 11517600 $
Chúc bạn học tốt @@