1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
= (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) + 5
= 10 + 10 + 10+ 10 + 5
= 45
10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90
= (10 + 90) + (20 + 80) + (30 + 70) + (40 + 60) + 50
= 100 + 100 + 100 + 100 + 50
= 450
1237 + 2249 + 1763 + 2251
= (1237 + 1763) + (2249 + 2251)
= 3000 + 4500
= 7500
1+2+3+4+5+6+7+8+9= 45
10+20+30+40+50+60+70+80+90 = 450
1237+2249+1763+2251 = 7500
Gộp từng cặp lại sao cho tổng tròn chục, tròn trăm, tròn nghìn
**1+2+3+4+5+6+7+8+9
Dãy trên có (9-1)/1+1=9 số hạng
Suy ra 1+2+3+4+5+6+7+8+9
=(9+1)*9/2=45
**10+20+30+40+50+60+70+80+90
Dãy trên có (90-10)/10+1=11 số hạng
Suy ra 10+20+30+40+50+60+70+80+90
=(90+10)*11/2=550
***1237+2249+1763+2251=(1237+2249)+(1763+2251)
=3486+4014
=7500
a/ 1+2+3+4+5+6+7+8+9
=(1+9)+(2+8)+(3+7)+(4+6)+5
=10+10+10+10+5
=45
b/ 10+20+30+40+50+60+70+80+90
=(90+10)+(20+80)+(30+70)+(40+60)+50
=100+100+100+100+50
=450
c/ 1237+2249+1763+2251
=(1237+1763)+(2249+2251)
=3000+4500
=7500
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 5
= 10 + 10 + 10 + 10 + 5
= 40 + 5
= 45
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
1237 + 2249 + 1763 + 2251
= ( 1237 + 1763 ) + ( 2249 + 2251 )
= 3000 + 4500
= 7500
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9= (1 + 9) + (2 + 8) + ( 3 + 7) + ( 4+6) + 5
= 10 + 10 + 10 + 10 + 5
= 45
10+20+30+40+50+60+70+80+90= (10 + 90) + (20 + 80) + ( 30 + 70) + ( 40+60) + 50
= 100 + 100 + 100 + 100 + 50
= 450
1237+2249+1763+2251= ( 1257 + 1763) + (2249 + 2251)
= 3000 + 4500
= 7500
(1+9)+...=
(10+90)+...=
(1237+1763)+(2249+2251)=
thong cam tự tính nhé
1+2+3+4+5+6+7+8+9= (1+9) + (2+8) + (3+7) + (4+6) +5 = 45
10+20+30+40+50+60+70+80+90= (10+90) + (20+80) + (30+70) + (40+60) +50 = 450
1237+2249+1763+2261= (1237 + 1763) + (2249 + 2261)
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
= (1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 5
= 10 + 10 + 10 + 10 + 5
= 10 x 4 + 5
= 40 + 5
= 45
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
1237 + 2249 + 1763 + 2251
= ( 1237 + 1763 ) + ( 2249 + 2251 )
= 3000 + 4500
= 7500
tính nhanh
1+2+3+4+5+6+7+8+9=45
10+20+30+40+50+60+70+80+90=450
1237+2249+1763+2251=7500
trả lời nha
3,72+2,05+6,28+7,95
=(3,72+6,28)+(2,05+7,95)
= 10 + 10
= 20