giúp mik vs 4/3.5 + 4/5.7 +..........+ 4/ 97.99 dấu"/" là phần nha

Lời giải:

$\frac{4}{3.5} + \frac{4}{5.7} +...+ \frac{4}{97.99}$ 

$= 2(\frac{2}{3.5} + \frac{2}{5.7} +... +\frac{2}{97.99})$ 

$= 2(\frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} +...+ \frac{1}{97} - \frac{1}{99})$ 

$= 2[\frac{1}{3} + (\frac{1}{5}-\frac{1}{5}) + (\frac{1}{7}-\frac{1}{7}) +...+ (\frac{1}{97}-\frac{1}{97}) - \frac{1}{99}]$ 

$= 2(\frac{1}{3} - \frac{1}{99})$ 

$= 2(\frac{33}{99} - \frac{1}{99})$ 

$= 2 . \frac{32}{99}$ 

$= \frac{64}{99}$