三年级奥数训练。请告诉我如何做这道题。

=(1000-2)+1413+(10000-11)

=1000-2+1413+10000-11

=1000+10000+1413-(2+11)

=12413-13

=12400

600-100-90-80-70-60-50-40-30-20-10

=600-100-(90+80+70+60+50+40+30+20+10)

=500-450

=50