s=1.2+2.3+3.4+...+99.100

Đáp án:

`S=1.2 + 2.3 + 3.4 + ...+ 99.100`
`3S=1.2.3+2.3.3+3.4.3+...+98.99.3+99.100.3`
`=1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)+99.100(101-98)`
`=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...-97.98.99+99.100.101-98.99.100`
`=99.100.101`

`= 3.33.100.101`

Đáp án:

`333300`

 Giải thích các bước giải:

$\text{S=1.2 + 2.3 + 3.4 + ...+ 99.100}$
$\text{3S=1.2.3+2.3.3+3.4.3+...+98.99.3+99.100.3}$
$\text{3S=1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)+99.100(101-98)}$
$\text{3S=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...-97.98.99+99.100.101-98.99.100}$
$\text{3S=99.100.101 3S = 3.33.100.101}$
→$\text{S=33.100.101= 333300}$