\(A=-\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{25.27}\right)-\frac{1}{27}\)

   \(=-\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{25}-\frac{1}{27}\right)-\frac{1}{27}\)

    \(=-\left(1-\frac{1}{27}\right)-\frac{1}{27}\)

    \(=-1+\frac{1}{27}-\frac{1}{27}\)

     \(=-1\)

sao làm như vậy được??? 2/19.21 và 2/23.25 bn k làm được như thế đâu vì nó k cùng quy luật với các ps kia

-1

Là -1.Đúng 100%

\(A=-\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{25.27}\right)-\dfrac{1}{27}\)

\(=-\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{25}-\dfrac{1}{27}\right)-\dfrac{1}{27}\)

\(=\)\(-\left(1-\dfrac{1}{27}\right)-\dfrac{1}{27}\)

\(=-1+\dfrac{1}{27}-\dfrac{1}{27}\)

\(=-1+\left(\dfrac{1}{27}-\dfrac{1}{27}\right)\)

\(=-1+0\)

\(=-1\)

A=\(\dfrac{2}{1.3}-\dfrac{2}{3.5}-\dfrac{2}{5.7}-.....-\dfrac{2}{23.25}-\dfrac{1}{27}\)

A=\(\dfrac{2}{3}-\left(\dfrac{2}{3.5}+\dfrac{2}{5.7}+....+\dfrac{2}{23.25}\right)-\dfrac{1}{27}\)

A=\(\dfrac{2}{3}-\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+......+\dfrac{1}{23}-\dfrac{1}{25}\right)-\dfrac{1}{27}\)

A=\(\dfrac{2}{3}-\left(\dfrac{1}{3}-\dfrac{1}{25}\right)-\dfrac{1}{27}\)

A=\(\dfrac{2}{3}-\dfrac{22}{75}-\dfrac{1}{27}\)

A=\(\dfrac{227}{675}\)

\(=-\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{25.27}\right)-\frac{2}{27}\)

\(=-\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{25}-\frac{1}{27}\right)-\frac{2}{27}\)

\(=-\left(1-\frac{1}{27}\right)-\frac{2}{27}\)

\(=-1+\frac{1}{27}-\frac{2}{27}\)

\(=-\frac{28}{27}\)

Bài này lớp 6 học rùi! 

S = 312/25

Bạn có cần giải cặn kẽ ko

ket qua la -1 ban a

minh vua lam xong! tik mik nhe

-1 nghe. đúng 100%