Bài $2$.
$a$) `S_{4} = 24 + 25 + 26 + ..... + 125 + 126`
`⇔ S_{1} = \frac{(126+24).[(126-24):1+1]}{2}`
`⇔ S_{1} = \frac{150.103}{2} = 7725`
$b$) `S_{5} = 1 + 4 + 7 + .... + 79`
`⇔ S_{2} = \frac{(79+1).[(79-1):3+1]}{2}`
`⇔ S_{2} = \frac{80.27}{2} = 1080`
$c$) `S_{6} = 15 + 17 + 19 + 21 + .... + 151 + 153 + 155`
`⇔ S_{6} = \frac{(155+15).[(155-15):2+1]}{2}`
`⇔ S_{6} = \frac{170.71}{2} = 6035`
$d$) `S_{7} = 15 + 25 + 35 + .... + 115`
`⇔ S_{7} = \frac{(115+15).[(115-15):10+1]}{2}`
`⇔ S_{7} = \frac{130.11}{2} = 715`