Đặt biểu thức đã cho là A
A= -( 1/15 + 1/21 + ...+1/120 - 1/10)
= -2(1/30 + 1/42 +...+ 1/240 - 1/20)
= -2( 1/5-1/6 + 1/6-1/7 + ...+ 1/15-1/16 - 1/4 + 1/5)
= -2 ( 1/5 -1/16 - 1/4 +1/5)
= -2( 2/5 -5/16)
= -2 . 7/80
= -7/40
Vậy A= -7/40
B=\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
tính:
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+.........+\frac{1}{120}\)
Tính nhanh: N = \(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
CỨU
\(\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}...\frac{1}{120}=\frac{5}{8}\)Tìm x
TINH\(C=\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-...-\frac{1}{120}=\frac{5}{8}\)
P=\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+..............+\frac{1}{120}\)
tìm x :
\(\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-....-\frac{1}{120}=\frac{5}{8}\)
