Tính
a, $\frac{-3}{2.5}$ -$\frac{3}{5.8}$ -….-$\frac{3}{302.305}$
b, $\frac{1}{99}$ – $\frac{1}{99.98}$ $\frac{1}{98.97}$ -…-$\frac{1}{3.2}$ -$\frac{1}{2.1}$
Tính
a, $\frac{-3}{2.5}$ -$\frac{3}{5.8}$ -….-$\frac{3}{302.305}$
b, $\frac{1}{99}$ – $\frac{1}{99.98}$ $\frac{1}{98.97}$ -…-$\frac{1}{3.2}$ -$\frac{1}{2.1}$
$a$) `-3/{2.5} – 3/{5.8} – … – 3/{302.305}`
`= -(3/{2.5} + 3/{5.8} + … + 3/{302.305})`
`= -(1/2 – 1/5 + 1/5 – 1/8 + … + 1/{302} – 1/{305})`
`= -(1/2 – 1/{305})`
`= -303/610`.
$b$) `1/{99} – 1/{99.98} – 1/{98.97} – …. – 1/{2.1}`
`= 1/99 – (1/{1.2} + …. + 1/{97.98} + 1/{98.99})`
`= 1/99 – (1/1 – 1/2 + …. + 1/{97} – 1/{98} + 1/{98} – 1/{99})`
`= 1/99 – (1 – 1/99)`
`= 1/99 – 1 + 1/99`
`= 2/99 – 1`
`= -97/99`.
Thấy hay thì cho tớ xin câu trả lời hay nhất nhá!
Bài làm
a) $\frac{-3}{x.5}$ – $\frac{3}{5.8}$ – ….. – $\frac{3}{302.305}$
= -( $\frac{3}{2.5}$ + $\frac{3}{5.8}$ +…..+ $\frac{3}{302.305}$ )
= -( $\frac{1}{2}$ – $\frac{1}{5}$ + $\frac{1}{5}$ – $\frac{1}{8}$ +…+ $\frac{1}{302}$ – $\frac{1}{305}$ )
= -( $\frac{1}{2}$ – $\frac{1}{305}$ )
= $\frac{-303}{610}
b) $\frac{1}{99}$ – $\frac{1}{99.98}$ – $\frac{1}{98.97}$ -….- $\frac{1}{3.2}$ – $\frac{1}{2.1}$
= $\frac{1}{99}$ – ( $\frac{1}{1.2}$ +….+ $\frac{1}{97.98}$ + $\frac{1}{98.99}$
= $\frac{1}{99}$ – ( 1 – $\frac{1}{99}$ )
= $\frac{1}{99}$ – 1 + $\frac{1}{99}$
= $\frac{-97}{99}$