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a) 15 cm2 + 20 cm2 = 35 cm2
60 cm2 - 42 cm2 = 18 cm2
20 cm2 + 10 cm2 + 15 cm2 = 45 cm2
b) 12 cm2 x 2 = 24 cm2
40 cm2 : 4 = 10 cm2
50 cm2 - 40 cm2 + 10 cm2 = 20 cm2
![](https://rs.olm.vn/images/avt/0.png?1311)
10+20+30+40+50+60+70+80+90+100=(10+100)+(20+90)+(30+80)+(40+70)+(50+60)
=110+110+110+110+110
=110x5
=550
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(10+20+30+40+50+60+70+80+90+100\)
\(=\left(10+90\right)+\left(20+80\right)+\left(30+70\right)+\left(40+60\right)+100+50\)
\(=100+100+100+100+100+50\)
\(=550\)
= ( 10 + 90 ) + ( 20 + 80) + ( 30 + 70 ) + ( 40 + 60 ) + 50 + 100
= 100 + 100 + 100 + 100 + 50 + 100
= 100 x 5 + 50
= 500 + 50
= 550
tk nhé
![](https://rs.olm.vn/images/avt/0.png?1311)
10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90
= ( 10 + 90 ) + ( 20 + 80 ) + ( 30 + 70 ) + ( 40 + 60 ) + 50
= 100 + 100 + 100 + 100 + 50
= 100 x 4 + 50
= 400 + 50
= 450
Học tốt #
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= (70 - 60) + (61 - 51) + (50 - 40) + (41 - 31) + ( 30 - 20) + (21 - 11) + 10
= 10 +10 + 10 + 10 + 10 + 10 + 10.
= 20 + 20 + 20 + 10
= 40 + 30
= 70
![](https://rs.olm.vn/images/avt/0.png?1311)
10+20+30+40+50+60+70+80+90+100
=(10+100)+(20+90)+(30+80)+(40+70)+(50+60)
= 110+110+110+110+110
=110x5
=550
![](https://rs.olm.vn/images/avt/0.png?1311)
Tính :
A = 1 + 2 + 3 + 4 + 5 + 5 + 6 + 7 + 8 + 9 + 10
A = ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + ( 5 + 5 ) + 10
A = 10 + 10 + 10 + 10 + 10 + 10
A = 60
B = 10 + 20 + 30 + 40 + 50 + 50 + 60 + 70 + 80 + 90 + 100
B = ( 10 + 90 ) + ( 20 + 80 ) + ( 30 + 70 ) + ( 40 + 60 ) + ( 50 + 50 ) + 100
B = 100 + 100 + 100 + 100 + 100 + 100
B = 600