\(=\left(\frac{1}{5}-\frac{1}{5}\right)+\left(\frac{3}{7}-\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(\frac{2}{11}-\frac{2}{11}\right)+\left(\frac{7}{13}-\frac{7}{13}\right)+\frac{9}{16}\)

= 0 + \(\frac{9}{16}\)

= \(\frac{9}{16}\)

tick nha bn

\(B=\frac{1}{5}-\frac{3}{7}+\frac{5}{9}-\frac{2}{11}+\frac{7}{13}-\frac{9}{16}-\frac{7}{13}+\frac{2}{11}-\frac{5}{9}+\frac{3}{7}-\frac{1}{5}\)

    \(=\left(\frac{1}{5}-\frac{1}{5}\right)+\left(-\frac{3}{7}+\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(-\frac{2}{11}+\frac{2}{11}\right)+\left(\frac{7}{13}-\frac{7}{13}\right)-\frac{9}{16}\)

    \(=0+0+0+0+0-\frac{9}{16}\)

    \(=-\frac{9}{16}\)

 ^...^  ^_^

cái này đăng hôm qua đến giờ 5 lần rồi,4 câu kia tui làm rồi vô chtt kiém coi có hk

\(A=\frac{1}{5}-\frac{3}{7}+\frac{5}{9}-\frac{2}{11}+\frac{7}{13}-\frac{9}{16}-\frac{7}{13}+\frac{2}{11}-\frac{5}{9}+\frac{3}{7}-\frac{1}{5}\)

\(=\left(\frac{1}{5}-\frac{1}{5}\right)+\left(-\frac{3}{7}+\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(-\frac{2}{11}+\frac{2}{11}\right)+\left(\frac{7}{13}-\frac{7}{13}\right)-\frac{9}{16}\)

\(=0+0+0+0+0+\frac{9}{16}=\frac{9}{16}\)

Chắc bạn gõ nhầm số hạng thứ 3 phải là +5/7.

Tổng này đối xứng qua 13/15. Các đối xứng trái dấu nên Tổng = 13/15

Tính theo công thức nha bạn

B=(1/5-1/5) + (-3/7+3/7) + (5/9-5/9) + (-2/11+2/11) + (7/13-7/13) - 9/16

  =     0       +       0       +      0      +         0         +      0         -  9/16

  =-9/16

bằng -9/16

B = 1 /5*(-3) /7 + 5 /9*(-2) /11 + 7 /13*(-9) /16*(-7) /13 + 2 /11*(-5) /9 + 3 /7*(-1) /5

B=-9/16

http://coccoc.com/search#query=T%C3%ADnh+nhanh+++B%3D+1%2F+5+%E2%88%92+3%2F+7+%2B+5%2F+9+%E2%88%92+2+%2F11+%2B+7%2F+13+%E2%88%92+9%2F+16+%E2%88%92+7+%2F13+%2B+2%2F+11+%E2%88%92+5%2F+9+%2B+3+%2F7+%E2%88%92+1%2F+5+

Ok

\(B=\frac{1}{5}-\frac{3}{7}+\frac{5}{9}-\frac{2}{11}+\frac{7}{13}-\frac{9}{16}-\frac{7}{13}+\frac{2}{12}-\frac{5}{9}+\frac{3}{7}-\frac{1}{5}-\frac{1}{5}\)

\(B=\left(\frac{1}{5}-\frac{1}{5}\right)-\left(\frac{3}{7}-\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(\frac{7}{13}-\frac{7}{13}\right)-\frac{2}{11}+\frac{2}{12}-\frac{9}{16}-\frac{1}{5}\)

\(B=0-0+0+0-\frac{2}{11}+\frac{2}{12}-\frac{9}{16}-\frac{1}{5}\)

\(B=\frac{-2}{11}+\frac{2}{12}-\frac{9}{16}-\frac{1}{5}\)

Đến đây chỉ còn cách quy đồng thôi

\(B=\left(\frac{1}{5}-\frac{1}{5}\right)+\left(-\frac{3}{7}+\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(-\frac{2}{11}+\frac{2}{11}\right)+\left(\frac{7}{13}-\frac{7}{13}\right)-\frac{9}{16}\)

\(B=0+0+0+0+0+\left(-\frac{9}{16}\right)=-\frac{9}{16}\)

\(B=\frac{1}{5}-\frac{3}{7}+\frac{5}{9}-\frac{2}{11}+\frac{7}{13}-\frac{9}{16}-\frac{7}{13}+\frac{2}{11}-\frac{5}{9}+\frac{7}{3}-\frac{1}{5}\)
\(B=\left(\frac{1}{5}-\frac{1}{5}\right)+\left(\frac{3}{7}-\frac{3}{7}\right)+\left(\frac{5}{9}-\frac{5}{9}\right)+\left(\frac{2}{11}-\frac{2}{11}\right)+\left(\frac{7}{11}-\frac{7}{11}\right)-\frac{9}{16}\)
\(B=0+0+0+0+0-\frac{9}{16}=-\frac{9}{16}\)