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@Mem gửi bạn
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$10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90$
$= ( 10 + 90 ) + ( 20 + 80 ) + ( 30 + 70 ) + ( 40 + 60 ) + 50$
$= 100 + 100 + 100 + 100 + 50$
$= 450$
10+20+30+40+50+60+70+80+90
= ( 10 + 90 ) + ( 20 + 80 ) + ( 30 + 70 ) + ( 40 + 60 ) + 50
= 100 + 100 + 100 + 100 + 50
= 100 × 4 + 50
= 400 + 50
= 450
