chung minh rang A=1/11×1/12×1/13+...+1/50>1/3
chung minh B=1/11+1/12+...+1/50>16/15
chứng minh: 1/11+1/12+...+1/50
a)>16/15
b) <8/3
1/11 + 1/12 + 1/13 + ... + 1/49 + 1/50 > 16/15
1/11 + 1/12 + 1/13 + ... + 1/49 + 1/50 > 8/3
So sánh A với 8/3 biết:
A = 1/11+1/12+1/13+.....+1/50
Đưa thừa số chung ra ngoài a, 5/7 . 5/11 + 5/7 . 2/11 - 5/7 . 14/11 b, 19 5/8 : 7/12 - 15 1/4 : 7/12 c, 2/5 . 1/3 - 2/15 : 1/5 + 3/5 . 1/3 d, 4/9 . 19 1/3 - 4/9 . 39 1/3
Chứng minh: \(M=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+...+\frac{1}{50}<\frac{8}{3}\)
Cho A=1/5^2+2/5^3+3/5^4+.........+n/5^n+1+........+11/5^12 với n thuoc N. Chung minh A>1/16
chung minh rang 11^n+2+12^2n+1 chia het cho 133
chung minh rang A=(17^n+1)(17^n+2)chia het cho 3 voi moi n thuoc N
cho (2a+7b) chia het cho 3 ( a b thuoc N). chung to (4a+2b) chia het cho 3