Vì A là bội của 69 nên A chia hết cho 69.

A không chia hết cho 69

Có nhầm lẫn j ko vậy bn??

mình không biết làm bài 2

A = \(\dfrac{1}{1.2}\) + \(\dfrac{1}{3.4}\) + \(\dfrac{1}{5.6}\)+....+ \(\dfrac{1}{49.50}\)

A = \(\dfrac{1}{1}\) - \(\dfrac{1}{2}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{6}\)+ \(\dfrac{1}{49}\) - \(\dfrac{1}{50}\)

A = 1 - \(\dfrac{1}{50}\) < 1

A = \(\dfrac{1}{1.2}\) + \(\dfrac{1}{3.4}\)+.....+ \(\dfrac{1}{49.50}\) < 1 ( đpcm)

Ta có: \(a=\frac{1}{1\cdot2}+\frac{1}{3\cdot4}+\cdots+\frac{1}{49\cdot50}\)

\(=1-\frac12+\frac13-\frac14+\cdots+\frac{1}{49}-\frac{1}{50}\)

\(=1+\frac12+\cdots+\frac{1}{50}-2\left(\frac12+\frac14+\cdots+\frac{1}{50}\right)\)

\(=1+\frac12+\cdots+\frac{1}{50}-1-\frac12-\cdots-\frac{1}{25}=\frac{1}{26}+\frac{1}{27}+\cdots+\frac{1}{50}\)

ta có: \(\frac{1}{26}<\frac{1}{25};\frac{1}{27}<\frac{1}{25};\ldots;\frac{1}{50}<\frac{1}{25}\)

Do đó: \(\frac{1}{26}+\frac{1}{27}+\cdots+\frac{1}{50}<\frac{1}{25}+\frac{1}{25}+\cdots+\frac{1}{25}\)

=>a<25/25

=>a<1