(1+2+3+4+····+1999+2000+1999+···+2+1)÷2000

(1+2+3+4+····+1999+2000+1999+···+2+1)÷2000

(1+2+3+4+····+1999+2000+1999+···+2+1)÷2000
=[2*(1+2+3+4+····+1999)+2000]/2000
=[(1+1999)*1999+2000]/2000
=(2000*2000)/2000
=2000