∵1/(2×5)=(1/2-1/5)/3
1/(5×8)=(1/5-1/8)/3
1/(8×11)=(1/8-1/11)/3
.
1/(2006×2009)=(1/2006-1/2009)/3
1/(2009×2012)=(1/2009-1/2012)/3
∴ 1/(2×5)+1/(5×8)+1/(8×11)+.+1/(2006×2009)+1/(2009×2012)
=(1/2-1/5)/3+(1/5-1/8)/3+(1/8-1/11)/3+.+(1/2006-1/2009)/3+(1/2009-1/2012)/3
=(1/2--1/2012)/3
=335/2012.