33390

45 phút

\(\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}\)

= \(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)

= \(\frac{1}{2}-\left(\frac{1}{3}-\frac{1}{3}-.....-\frac{1}{5}\right)-\frac{1}{6}\)

= \(\frac{1}{2}-\frac{1}{6}\)

= \(\frac{1}{3}\)

300

k cho mk đi bn

sau co bai j nho ban giai giup nha

A = 1/15 + 1/35 + 1/63 + 1/99 + ... + 1/255

A = 1/3x5 + 1/5x7 + 1/7x9 + 1/9x11 +...+ 1/15x17

2A = 2/3x5 + 2/5x7 + 2/7x9 + .... + 1/15x17

2A = 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + .... + 1/15 - 1/17

2A = 1/3 - 1/17 = 14/51

A = 14/51 : 2 = 7/51

 X = 121,27