a < 1

Ta thấy: 1/6<1/5

               1/7<1/5

               1/8<1/5

               1/9<1/5

=>A=1/5+1/6+1/7+1/8+1/9<1/5+1/5+1/5+1/5+1/5

=>A<5.1/5

=>A<1

Vậy A<1

có:1/4+1/5+1/6+1/7+...+1/9≤nhỏ hơn 1/6.6=1

1/10+1/11+...+1/15 nhỏ hơn1/5.5=1

⇒1/4+1/5+...+1/15nhỏ hơn1+1=2(đpcm)

ta có

\(\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+\dfrac{1}{7}< \dfrac{1}{4}.4\)

\(\Rightarrow\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+\dfrac{1}{7}< 1\)

và:

\(\dfrac{1}{8}+\dfrac{1}{9}+\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}+\dfrac{1}{13}+\dfrac{1}{14}+\dfrac{1}{15}< \dfrac{1}{8}.8\)

\(\dfrac{1}{8}+\dfrac{1}{9}+\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}+\dfrac{1}{13}+\dfrac{1}{14}+\dfrac{1}{15}< 1\)

\(\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+\dfrac{1}{7}+...+\dfrac{1}{15}< 1+1=2\)

tổng của A là

1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 = 223/140

=> 223/140 > 6/7

k mk nha

A < 1 

tick giùm nhé

bạ tự là đi minh mới lớp 6 nhá

a,\(A=\frac{1}{5}+\frac{1}{5^2}+\frac{1}{5^3}+...+\frac{1}{5^{100}}\)

\(=>5A=1+\frac{1}{5}+\frac{1}{5^2}+...+\frac{1}{5^{99}}\)

\(=>5A-A=1-\frac{1}{5^{100}}=>A=\frac{1-\frac{1}{5^{100}}}{4}\)

b, Ta có \(1-\frac{1}{5^{100}}< 1=>\frac{1-\frac{1}{5^{100}}}{4}< \frac{1}{4}\)hay \(A< \frac{1}{4}\)