M=1/3x4/6x7/9x...x208/210
CTR:M<1/25
A=1/3x4/6x7/9x10/12x ... x 208/210 cmr a<1/25
cho s biet:1/101+1/102+....+1/130. chung minh rang 1/4<s<91/330
Cho S=1/3+1/3^2+1/3^3+...+1/3^2015 Chung minh rang s<1/2
cho s =3/1*4+3/4*7+3/7*10+......+3/43*46.chung minh rang s <1
chung minh rang
1/1x2+1/3x4+1/5x6....+1/49x50=1/26+1/27+1/28....+1/50
cho s=1 phan 11+1 phan 12+1phan13+.....+1phan19+1phan20
chung minh rang 1phan2 lon hon S lon hon 1
Cho S= 3/1+3/11+3/12+3/13+3/14
Chung minh rang 1<S<2
cho S = 1+3+32+ 33 + 34 + .......+ 399
a) Chung minh rang S chia het cho 4
b) chung minh rang S chia het cho 40
giup minh nhek
