1/n+2/n+.+(n-1)/n=(n-1)/2
1/2+(1/3+2/3)+(1/4+2/4+3/4)+…+(1/2002+2/2002+…+2001/2002
=(1+2+.+2001)/2
=(1+2001)×2001÷(2×2)
=2003001/2