s=3/10+3/11+3/12+3/13+3/14>3/15+3/15+3/15+3/15+3/15=1 mak3/15+3/15+3/15+3/15+3/15<3/10+3/10+3/10+3/10+3/10<20/10=2

=>1<S<2=>s ko fải stn

CHÚC BẠN HK TỐT

SAI THÌ MK SR NHA^^

vi 3/10>3/15

3/11>3/15

...........

3/14>3/15

cong ve ta duoc

3/10+3/11+...+3/14>3/15.5=1(1)

vi 3/10<3/9

3/11<3/9

..............

3/14<3/9

cong ve ta duoc

3/10+3/11+...+3/14<3/9.5=5/3<2(2)

tu (1)va(2)

=>1<A<2

=>A khong phai la so tu nhien

+TH₁: \(M>1\)

 Ta có: \(\frac{3}{10}>\frac{3}{15}=\frac{1}{5}\)

            \(\frac{3}{11}>\frac{3}{15}=\frac{1}{5}\)

            \(\frac{3}{12}>\frac{3}{15}=\frac{1}{5}\)

            \(\frac{3}{13}>\frac{3}{15}=\frac{1}{5}\)

            \(\frac{3}{14}>\frac{3}{15}=\frac{1}{5}\)

 \(\Rightarrow M=\frac{3}{10}+\frac{3}{11}+\frac{3}{12}+\frac{3}{13}+\frac{3}{14}>\frac{1}{5}+\frac{1}{5}+\frac{1}{5}+\frac{1}{5}+\frac{1}{5}=5.\frac{1}{5}=1\)

 \(\Rightarrow M>1\) 1

+TH₂: \(M< 2\)

 Ta có: \(\frac{3}{10}< \frac{3}{9}=\frac{1}{3}\)

           \(\frac{3}{11}< \frac{3}{9}=\frac{1}{3}\)

           \(\frac{3}{12}< \frac{3}{9}=\frac{1}{3}\)

           \(\frac{3}{13}< \frac{3}{9}=\frac{1}{3}\)

           \(\frac{3}{14}< \frac{3}{9}=\frac{1}{3}\)

 \(\Rightarrow M=\frac{3}{10}+\frac{3}{11}+\frac{3}{12}+\frac{3}{13}+\frac{3}{14}< \frac{1}{3}\frac{1}{3}\frac{1}{3}\frac{1}{3}\frac{1}{3}=\frac{5}{3}< \frac{6}{3}=2\)

 \(\Rightarrow M< 2\) 2

Từ 1 và 2 \(\Rightarrow1< M< 2\)hay M không phải là số tự nhiên