có thể đây là bài lớp 4 nhưng mình nghĩ là các bạn lớp 5 cũng sẽ khó khăn đó
dành cho các bn học sinh giỏi
1 ,
1/ 1*2 + 1/ 2*3 + 1 / 3*4 + ... + 1/ 9*10
= 1/1 - 1/2 + 1/2 - 1/3 + 1/3-1/4 + ... + 1/9 - 1/10
= 1/1 - 1/10
= 9/10
2 , gọi A= 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56
A= 1/ 2*3 + 1/3*4 + 1/4*5 + 1/5*6 + 1/6*7 + 1/7*8
A= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8
A= 1/2 - 1/8
A= 3/8
Bài làm :
Bài 1 :
Ta có :
\(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+...+\frac{1}{8\times9}+\frac{1}{9\times10}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}\)
\(=\frac{9}{10}\)
Bài 2 :
Ta có :
\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(=\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{7\times8}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-...-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)
bài 1: =1-\(\frac{1}{2}\)+\(\frac{1}{2}\)-\(\frac{1}{3}\)+\(\frac{1}{3}\)-\(\frac{1}{4}\)+...+\(\frac{1}{9}\)-\(\frac{1}{10}\)=1-\(\frac{1}{10}\)=\(\frac{9}{10}\) bài 2: =\(\frac{1}{2x3}\)+\(\frac{1}{3x4}\)+\(\frac{1}{4x5}\)+\(\frac{1}{5x6}\)+\(\frac{1}{6x7}\)+\(\frac{1}{7x8}\)=\(\frac{1}{2}\)x(\(\frac{1}{2}\)-\(\frac{1}{3}\)+\(\frac{1}{3}\) -\(\frac{1}{4}\) +...+\(\frac{1}{7}\) -\(\frac{1}{8}\))=\(\frac{1}{2}\)x(\(\frac{1}{2}\)-\(\frac{1}{8}\))=\(\frac{1}{2}\)x\(\frac{3}{8}\)=\(\frac{3}{16}\)
bài1=9/10
bài2=3/8
