\(A=\dfrac{1}{1^2}+\dfrac{1}{2^2}+\dfrac{1}{3^2}+...+\dfrac{1}{50^2}=\dfrac{1}{1.1}+\dfrac{1}{2.2}+\dfrac{1}{3.3}+...+\dfrac{1}{50.50}\)\(A=\dfrac{1}{1.1}+\dfrac{1}{2.2}+\dfrac{1}{3.3}+...+\dfrac{1}{50.50}< \dfrac{1}{1.1}+\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{49.50}\left(1\right)\)Mà :\(\dfrac{1}{1}+\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{49.50}=\dfrac{1}{1}+\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{49}-\dfrac{1}{50}\)
\(=1+1-\dfrac{1}{50}=1+\dfrac{49}{50}=\dfrac{99}{50}< \dfrac{100}{50}=\dfrac{1}{2}\left(2\right)\)
Từ (1) và (2) ta suy ra A<2
B có 30 số hạng, chia B thành 5 nhóm, mỗi nhóm có 6 số hạng như sau:
\(B=\left(2^1+2^2+2^3+2^4+2^5+2^6\right)+\left(2^7+2^8+2^9+2^{10}+2^{11}+2^{12}\right)+...+\left(2^{25}+2^{26}+2^{27}+2^{28}+2^{29}+2^{30}\right)\)
\(B=2^1\left(1+2+2^2+2^3+2^4+2^5\right)+2^7\left(1+2+2^2+2^3+2^4+2^5\right)+...+2^{25}\left(1+2+2^2+2^3+2^4+2^5\right)\)
\(B=2^1.63+2^7.63+...+2^{25}.63\)
\(B=63.\left(2^1+2^7+...+2^{25}\right)⋮63\)
\(B=21.3.\left(2^1+2^7+...+2^{25}\right)⋮21\left(đpcm\right)\)
bạn tách số là đc số đẹp