1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20
= (1 +19) + (2 + 18) + (3 + 17) + (4 + 16) + (5 + 15) + (6 + 14) + (7 + 13) + (8 + 12) + (9 + 11) + 10 + 20
= 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 10
= 20 x 10 + 10
= 200 + 10
= 210
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20=210 nhe!
dễ thế mà ko biết
= (1+19) + (2+18) + (3+17) + (4+16) + (5+15) + (4+16) + (3+17) + (2+18) + (1+19) + 10 + 20 = 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 10 + 20 = 20 x 10 + 10 =200 + 10 = 210
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20
= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) + ( 4 + 16 ) + ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12 ) + ( 9 + 11 ) + 10 + 20
= 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 10 + 20
= 20 x 10 + 10
= 200 + 10
= 210