\(S=\frac{2}{20}+\frac{2}{30}+...+\frac{2}{240}\)
\(=2\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{15.16}\right)\)
\(=2\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{15}-\frac{1}{16}\right)\)
\(=2\left(\frac{1}{4}-\frac{1}{16}\right)\)
\(=2\times\frac{3}{16}\)
\(=\frac{3}{8}\)
cho M = 1/10 + 1/15 + 1/21 + 1/28 + ....+ 1/105 + 1 /120.Chung to 1/3 < M < 1/2
a, \(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{1}{306}\)
b, \(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
Tính tổng các phân số trên
TÌM X BIẾT :
x/2017-1/10-1/15-1/21-1/28-1/36-1/45-1/55-1/66-1/78-1/91-1/105-1/120=5/8
1 Tính: A=1/10+1/15+1/21/+......+1/120
Tính A=1/10 + 1/15 + 1/21 +...+1/120
Tính D = 1/10 + 1/15 + 1/21 + ... + 1/120
Tính:
B= 1/10 + 1/15 + 1/21 + .... +1/120
tính nhanh:
C = 1/10 + 1/15 + 1/21 +.......+ 1/120
1/6+1/10+1/15+1/21+1/28+/36+1/45 (Tính tổng)