（x/1×2）+（x/2×3）+（x/3×4）+ … +（x/2008×2009）=2008
x(1/1×2+1/2×3+1/3×4+ … +1/2008×2009)=2008
x(1-1/2+1/2-1/3+1/3-1/4+ … +1/2008-1/2009)=2008
x(1-1/2009)=2008
2008x/2009=2008
x=2008÷2008/2009
x=2009