Question

tính nhanh:1/30+1/42+1/56+1/72...+1/210= bao nhiêu

giải ra 

ai nhanh nhất mk k cho nha

Answer 1

mk ko chép đề nha

Answer 2

1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 . ..... . 1/14.15

= 1/5 + 1/6 - 1/6 + 1/7 + 1/7 - 1/8 + 1/9 - ..... + 1/14 - 1/15

= 1/5 - 1/15

= 2/15

Answer 3

\(A=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)

\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)

\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{14}-\frac{1}{15}\)

\(A=\frac{1}{5}-\frac{1}{15}\)

\(A=\frac{2}{15}\)

Answer 4

  1/30 + 1/42 + 1/56 + 1/72 + ....+1/210

=1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + .... + 1/14.15

=1/5-1/6+1/6-1/7+1/7-1/8+.....+1/14-1/15

=1/5-1/15

=2/15

Tk mình nha!

Answer 5

\(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)

\(=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)

\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{14}-\frac{1}{15}\)

\(=\frac{1}{5}+\left(\frac{1}{6}-\frac{1}{6}\right)+\left(\frac{1}{7}-\frac{1}{7}\right)+\left(\frac{1}{8}-\frac{1}{8}\right)+...+\left(\frac{1}{14}-\frac{1}{14}\right)-\frac{1}{15}\)

\(=\frac{1}{5}+0+0+0+...+0-\frac{1}{15}\)

\(=\frac{1}{5}-\frac{1}{15}\)

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

Answer 6

\(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)

\(=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)

\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{14}-\frac{1}{15}\)

\(=\frac{1}{5}+\left(-\frac{1}{6}+\frac{1}{6}\right)+\left(-\frac{1}{7}+\frac{1}{7}\right)+\left(-\frac{1}{8}+\frac{1}{8}\right)+...+\left(-\frac{1}{14}+\frac{1}{14}\right)-\frac{1}{15}\)

\(=\frac{1}{5}+0+0+0+...+0-\frac{1}{15}\)

\(=\frac{1}{5}-\frac{1}{15}=\frac{3}{15}-\frac{1}{15}=\frac{2}{15}\)

Vậy : \(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{210}=\frac{2}{15}\)