minh cần câu trả loi nhanh nhất có thể

Đặt \(A=\dfrac{1}{3}+\dfrac{1}{31}+\dfrac{1}{35}+\dfrac{1}{37}+\dfrac{1}{47}+\dfrac{1}{53}+\dfrac{1}{61}\)

\(A< \left(\dfrac{1}{30}+\dfrac{1}{30}+\dfrac{1}{30}\right)+\left(\dfrac{1}{60}+\dfrac{1}{60}+\dfrac{1}{60}+\dfrac{1}{60}\right)\)

\(A< \dfrac{1}{3}+\dfrac{3}{30}+\dfrac{4}{60}\)

\(A< \dfrac{10}{30}+\dfrac{3}{30}+\dfrac{2}{30}\)

\(A< \dfrac{15}{30}=\dfrac{1}{2}\)

\(\Rightarrow A< \dfrac{1}{2}\) ( đpcm ).

Ta có: Gọi dãy số cần chứng minh là A

A<(130 +130 +130 )+(160 +160 +160 +160 )

A<13 +330 +460 

A<1030 +330 +230 

A<1330 +230 

A<1530 =12 

Vậy A<12 

1/3+1/31+1/35+1/37+1/47+1/53+1/61 < 1 / 3 + 3 / 31 + 3 / 47 < 1 / 3 + 3 / 30 + 3 / 45 = 
1 / 3 + 1 / 10 + 1 / 15 = 1 / 3 + (1 / 30) * (3 + 2) = 1 / 3 + (1 / 30) * 5 = 1 / 3 + 1 / 6 = 
(1 / 6) * (2 + 1) = (1 / 6) * 3 = 1 / 2 
 

1/3+1/31+1/35+1/37+1/47+1/53+1/61 < 1 / 3 + 3 / 31 + 3 / 47 < 1 / 3 + 3 / 30 + 3 / 45 = 
1 / 3 + 1 / 10 + 1 / 15 = 1 / 3 + (1 / 30) * (3 + 2) = 1 / 3 + (1 / 30) * 5 = 1 / 3 + 1 / 6 = 
(1 / 6) * (2 + 1) = (1 / 6) * 3 = 1 / 2

toi chua biet moi hoi

Nhớ k cho mình nhé bạn 

Ta có: Gọi dãy số cần chứng minh là A

\(A<\left(\frac{1}{30}+\frac{1}{30}+\frac{1}{30}\right)+\left(\frac{1}{60}+\frac{1}{60}+\frac{1}{60}+\frac{1}{60}\right)\)

\(A<\frac{1}{3}+\frac{3}{30}+\frac{4}{60}\)

\(A<\frac{10}{30}+\frac{3}{30}+\frac{2}{30}\)\(\)

\(A<\frac{13}{30}+\frac{2}{30}\)

\(A<\frac{15}{30}=\frac{1}{2}\)

Vậy \(A<\frac{1}{2}\)

xet hieu 1/3+1/31+1/35+1/47+1/53+1/61-1/2=-0.04929921068luon nho hon 1/2 suy ra dieu phai chung minh