\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+..+\frac{2}{399}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)
\(=\frac{1}{3}-\frac{1}{21}=\frac{7}{21}-\frac{1}{21}=\frac{6}{21}=\frac{2}{7}\)
Ủng hộ mk nha !!! ^_^
B = 2/15 + 2/35 + 2/63 +...+ 2/399
Bài 1: tính
A=2/15 +2/35 + 2/63 +.......+ 2/399
A=\(\dfrac{2}{15}\)+\(\dfrac{2}{35}\)+\(\dfrac{2}{63}\)+...+\(\dfrac{2}{399}\)
Tính tổng:
D=3-32-33-..........-3100
B=-2/15-2/35-2/63-..........-2/399
A=1/3+1/6+1/10+1/15+...+1/66
B= 2/15+2/35+2/63+..+2/399
A=\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
C=\(\frac{2}{15}\)+\(\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}=?\)
a) A= 4/3.7 + 4/7.11 + 4/11.15 +...+ 4/107.111
b) B= 2/15 + 2/35 +2/63+...+ 2/399
c) C= 1/10 + 1/15 + 1/21+ ...+ 1/120