Đặt A = 1+2+3+4+...+100000000 (có 100000000 số hạng)
A = (1+100000000) x 100000000 : 2
A = 100000001 x 100000000 : 2
A = 10000001 x 50000000
A = 5000000500000000
Vậy 1+2+3+4+...+100000000 = 5000000500000000
Đặt A = 1+2+3+4+...+100000000 (có 100000000 số hạng)
A = (1+100000000) x 100000000 : 2
A = 100000001 x 100000000 : 2
A = 10000001 x 50000000
A = 5000000500000000
Vậy 1+2+3+4+...+100000000 = 5000000500000000