[Siren-Of-Step]

ข้อ 18
จะได้ว่า $a_1 = 1 , a_2 = \frac{1}{2} , ...... , a_{2010} = \frac{1}{2010}$
$a_1a_2 + a_2a_3+a_3a_4 + ....+ a_{2009}a_{2010} = \frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...............+\frac{1}{2009}-\frac{1}{2010} = \frac{2009}{2010} $