\(\Rightarrow A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
\(\Rightarrow A=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
\(\Rightarrow A=1-\frac{1}{7}=\frac{6}{7}\)
A=1/2 + 1/6 + 1/12 + 1/20 + 1/30 +1/42
A=1/1x2 + 1/2x3 +1/ 3x4 + 1/4x5 + 1/5x6 + 1/6x7
Nhận xét : 1/1x2=1/1-1/2 1/2x3=1/2-1/3 1/6x7=1/6-1/7
Ta có :
A= 1/1x2 +1/2x3 + 1/3x4 + 1/4/5 + 1/5x6 + 1/6x7
A= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6- 1/7
A=1/1 - 1/7
A=6/7
Vậy A=6/7
duyệt đi
A = 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7
A = 1/1 - 1/2 + 1/2 -1/3 + 1/3 -1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 ( tách ra )
A = 1/1 - 1/7 = 6/7
bài này là toán nâng cao dạng dễ nên ko khó đâu bạn
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/5 - 1/6 + 1/6 - 1/7
A =1/1 - 1/7
A =1 - 1/7 = 6/7
A = 6/7
