dễ lm bạn ơi

B=4/3.5+4/5.7+.........................

Ghi tới đok chắc hỉu roài chứ, tự lm phần sau nhé.

bài này bằng -216/455 nha

thanh tịnh sai rồi câu này = \(\frac{-20}{39}\)

B = 1/4 + 1/15 + 1/35 + 1/63 + 1/99 + 1/143 + 1/195

= 1/4 + 1/(3.5) + 1/(5.7) + 1/(7.9) + 1/(9.11) + 1/(11.13) + 1/(13.15)

= 1/4 + 1/2.(1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13 + 1/13 - 1/15)

= 1/4 + 1/2.(1/3 - 1/15)

= 1/4 + 1/2 . 4/15

= 1/4 + 2/15

= 23/60

4/15 + 4/35 + 4/63 + 4/99 + 4/143 = 20/39

\(B=\frac{4}{35}+\frac{4}{63}+\frac{4}{99}+\frac{4}{143}+\frac{4}{195}\)

\(B=2\cdot\left(\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+\frac{2}{9\cdot11}+\frac{2}{11\cdot13}+\frac{2}{13\cdot15}\right)\)

\(B=2\cdot\left(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}\right)\)

\(B=2\cdot\left(\frac{1}{5}-\frac{1}{15}\right)\)

\(B=2\cdot\frac{2}{15}\)

\(B=\frac{4}{15}\)

B = 4/35 + 4/63 + 4/99 + 4/143 + 4/195

B = 4/5.7 + 4/7.9 + 4/9.11 + 4/11.13 + 4/13.15

B = 4/2 . ( 2/5.7 + 2/7.9 + 2/9.11 + 2/11.13 + 2/13.15 )

B = 4/2 . ( 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13 + 1/13 - 1/15 )

B = 4/2 . ( 1/5 - 1/15 )

B = 4/2 . 2/15

B = 4/15

4/15 + 4/35 + 4/63 + 4/99 + 4/143

= 8/21 + 8/77 + 4/143

= 16/33 + 4/143

= 20/39

\(\frac{4}{15}+\frac{4}{35}+\frac{4}{63}+\frac{4}{99}+\frac{4}{143}\)

\(=2\times\left(\frac{1}{3\times5}+\frac{1}{5\times7}+...+\frac{1}{11\times13}\right)\)

\(=2\times\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{13}\right)\)

\(=2\times\left(\frac{1}{3}-\frac{1}{13}\right)\)

\(=2\times\frac{10}{39}\)

\(=\frac{20}{39}\)