990+991+992……1007+1008+1009
=1+2+3+.+989+.+1007+1008+1009-(1+2+3+.989)
=(1+1009)×1009÷2-(1+989)×989÷2
=505×1009-495×989
=505×(1000+10-1)-495×(1000-10-1)
=505000+5050-505-495000+4950+495
=10000+10000-10
=19990