Question

[Cute Kittens]

Tính tổng:

a> 2/1x3 + 2/3x5 + 2/5x7 +2/7x9

b> 1/3 + 1/15 +  1/35 + 1/63 + .........+ 1/195

Giups mình với! Mình cảm ơn các bạn nhìu!

Answer 1

[Nguyễn Thị Thương Hoài]

A = \(\dfrac{2}{1\times3}\) + \(\dfrac{2}{3\times5}\) + \(\dfrac{2}{5\times7}\) + \(\dfrac{2}{7\times9}\)

A = \(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}\) + \(\dfrac{1}{7}-\dfrac{1}{9}\)

A = \(\dfrac{1}{1}-\dfrac{1}{9}\)

A = \(\dfrac{8}{9}\)

Answer 2

[Nguyễn Thị Thương Hoài]

B = \(\dfrac{1}{3}+\dfrac{1}{15}\) + \(\dfrac{1}{35}+\) \(\dfrac{1}{63}\) + ... + \(\dfrac{1}{195}\)

B = \(\dfrac{1}{1\times3}\) + \(\dfrac{1}{3\times5}\) + \(\dfrac{1}{5\times7}\) + ...+ \(\dfrac{1}{13\times15}\)

B = \(\dfrac{1}{2}\) x (\(\dfrac{2}{1\times3}\) + \(\dfrac{2}{3\times5}\) + \(\dfrac{2}{5\times7}\) + ..+ \(\dfrac{1}{13}\) - \(\dfrac{1}{15}\))

B = \(\dfrac{1}{2}\) x (\(\dfrac{1}{1}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}-\dfrac{1}{5}\) + ...+\(\dfrac{1}{13}-\dfrac{1}{15}\))

B = \(\dfrac{1}{2}\) x (\(\dfrac{1}{1}-\dfrac{1}{15}\))

B = \(\dfrac{1}{2}\) x \(\dfrac{14}{15}\)

B = \(\dfrac{7}{15}\)