10 + 20 + 30 + 40 + 50 + 60 + 70 +80 + 90  

=(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) x 10

=45 x 10

=450

10+20+30+40+50+60+70+80+90

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

=100+100+100+100+50

=400+50

=450

\(10+20+30+40+50+60+70+80+90\)

\(=\left(10+90\right)+\left(20+80\right)+\left(30+70\right)+\left(40+60\right)+50\)

\(=100+100+100+100+50\)

\(=400+50\)

\(=450\)