nguyễn thị kim ngân
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2023-04-09 14:17:46
đáp án : cơ bắp
2023-04-07 21:45:43
Ta có : 1-1/2+1/3-1/4+1/5-1/6
= 1+1/2+1/3+1/4+1/5+1/6-2.(1/2+1/4+1/6)
= 1+1/2+1/3+1/4+1/5+1/6-(1+1/2+1/3)
=1+1/2+1/3+1/4+1/5+1/6-1-1/2-1/3
=1/4+1/5+1/6
2023-04-05 23:00:53
Ta có :22A=1+\(\dfrac{1}{2^2}\)+\(\dfrac{1}{4^2}\)+...+\(\dfrac{1}{n^2}\)
22A-A=1-\(\dfrac{1}{\left(2n\right)^2}\)
3A=\(\dfrac{\left(2n\right)^2-1}{\left(2n\right)^2}\) <\(\dfrac{n^2}{\left(2n\right)^2}\)=\(\dfrac{1}{2}\)
3A<\(\dfrac{1}{2}\) suy ra A<\(\dfrac{1}{2}\)