Giải:
Đặt A = 1/3+1/15+1/35+1/63+1/99+1/143+1/195
2A= 2/(1.3) + 2/(3.5) + 2/(5.7) + 2/(7.9)+2/(9.11) + 2/(11.13)+2/(13.15)
2A=1/1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9...
2A=1/1-1/15=14/15
Vậy A=14/15 : 2 = 7/15
Nhấn đúng mk nha 
Tran Dan
\(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+..+\frac{1}{143}+\frac{1}{195}\)
=\(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+..+\frac{1}{13.15}\)
= \(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+..+\frac{1}{13}-\frac{1}{15}\)
= \(1-\frac{1}{15}=\frac{14}{15}\)
tick đúng nha
A = \(\frac{1}{1\times3}+\frac{1}{3\times5}+\frac{1}{5\times7}+...+\frac{1}{13\times15}=\frac{1}{2}\times\left(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{13\times15}\right)\)
A = \(\frac{1}{2}\times\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{13}-\frac{1}{15}\right)=\frac{1}{2}\times\left(1-\frac{1}{15}\right)=\frac{7}{15}\)
\(\text{Đặt:}\)\(A=\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+...+\frac{1}{195}\)
\(\Leftrightarrow A=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+.....+\frac{1}{13.15}\)
\(\Rightarrow2A=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+.....+\frac{2}{13.15}\)
\(\Rightarrow2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+.......+\frac{1}{13}-\)
\(\Rightarrow2A=1-\frac{1}{15}\)
\(\Rightarrow2A=\frac{14}{15}\)
\(\Rightarrow A=\frac{14}{15}:2=\frac{7}{15}\)
cap 2 cung co bai tap nay . Hog biet lam the nao
Bài này dễ lắm luôn á. Lớp 4 còn làm được
A = 1/3+1/15+1/35+1/63+1/99+1/143+1/195=0.46666666666
#HT#
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