Ta có : \(A>\frac{1}{3^2}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{50.51}\)
\(\rightarrow A>\frac{1}{9}+\frac{1}{4}-\frac{1}{4}+\frac{1}{5}-\frac{1}{5}+...+\frac{1}{50}-\frac{1}{50}-\frac{1}{51}\)
\(\rightarrow A>\frac{1}{4}+\left(\frac{1}{9}-\frac{1}{51}\right)\)
Xét : \(\frac{1}{9}-\frac{1}{51}>0\rightarrow A>\frac{1}{4}\left(đpcm\right)\)
CHo A=1/3^2+1/4^2+1/5^2+...+1/50^2. Chứng tỏ rằng 1/4<A<4/9
Cho A = 1/32+1/42+1/52+.....+1/502 . Chứng minh rằng :
a) A > 1/4
b) A < 4/9
Cho A = 1/3 mũ 2 +1/4 mũ 2 +...+1/50 mũ 2. Chứng minh rằng 1/4 < A < 4
Cho A=1-1/2+1/3-1/4+1/5-1/6+...+1/49-1/50. Chứng minh rằng: A<5/6
Cho A =1-1/2+1/3-1/4+...+1/49-1/50 Hãy chứng tỏ rằng 7/12<A<5/6
Cho A=1/2^2+1/3^2+...+1/50^2.Chứng minh rằng a>1/4
A=1/3^2+1/4^2+1/5^2+.................+1/50^2
Chứng minh rằng: a)A>1/4 b)A<4/9
Cho A=1+1/2+1/3+1/4+...+1/2^100-1.Chứng tỏ rằng 50<A<100
