cho A =1/2 + 1/3 +...+1/50
Cho C = 49/1+48/2+...+1/49=50.A
CHung TO C KO PHAI LA SO TU NHIEN
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chung minh C =49/1 +48/2 +...+2/48 +1/49 khong phai la so tu nhien
cho A = 1/2 + 1/3 + 1/4 + ... +1/50
CHO C = 49/1 + 48/2 + 47/3 +...+ 2/48 + 1/49 = 50.A
chứng tỏ C không phải là số tự nhien
=> \(A=\frac{\left(\frac{49}{1}+\frac{48}{2}+...+\frac{1}{49}\right)}{50}=\frac{49}{50.1}+\frac{48}{50.2}+...+\frac{1}{50.49}\)
=> \(A=\frac{50-1}{50.1}+\frac{50-2}{50.2}+...+\frac{50-49}{50.49}\)
=> \(A=\left(\frac{50}{50.1}+\frac{50}{50.2}+...+\frac{50}{50.49}\right)-\left(\frac{1}{50.1}+\frac{2}{50.2}+...+\frac{49}{50.49}\right)\)
=> \(A=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{49}\right)-\left(\frac{1}{50}+\frac{1}{50}+...+\frac{1}{50}\right)\) ( có 49 số 1/50 )
=> \(A=1+\frac{1}{2}+...+\frac{1}{49}-\frac{49}{50}=\left(1-\frac{49}{50}\right)+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{49}\)
=> \(A=\frac{1}{2}+\frac{1}{3}+...+\frac{1}{50}\)
Vậy A không phải là số tự nhiên
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chung minh rang A = 1/2 + 1/3 + 1/4 +...+ 1/50 khong phai la so tu nhien
Cho S =1+3+3^2+3^3+.........+3^48+3^49
a)Chung to S chia het cho 4
b)Tim chu so tan cung cua S
c)Chung to S=3^50-1/2
chung to rang 1 tren 1 mu 2 + 1 tren 1 mu 3+...+1 tren n mu 2( voi n thuoc N sao) ko phai la mot so tu nhien
cho so tu nhien a gom 100 chu so va 1 so tu nhien b gom 50 chu so 2 chung minh ava b la hai so chinh phuong
Voi n la so tu nhien, cho: A=20134n(2013(4n+3)-1)-1. Chung minh rang A khong phai la so nguyen to.
Tinh ti so a/b
A= 1/2+1/3+1/4+.....+1/48+1/49+1/50
B= 1/49+2/48+3/47+.....+48/2+49/1
Cho
A=1/2+1/3+...+1/49+1/50
B=1/49+2/48+...+48/2+49/2