Chứng tỏ rằng :
a) \(\frac{11}{15}<\frac{1}{21}+\frac{1}{22}+\frac{1}{23}+...+\frac{1}{60}<\frac{3}{2}\)
Giúp mik với
Cho A=\(\frac{10}{17}+\frac{8}{15}+\frac{11}{16}\).Chứng tỏ rằng A< 2
Cho A = 10/17 + 8/15 + 11/16. Chứng tỏ rằng A < 2
CHO:
\(\frac{17}{10}+\frac{8}{15}+\frac{11}{6}\)
Chứng tỏ A<2
Bài 1 :Chứng tỏ rằng :
a) \(\frac{11}{15}< \frac{1}{21}+\frac{1}{22}+\frac{1}{23}+...+\frac{1}{59}+\frac{1}{60}< \frac{3}{2}\)
b) \(3< 1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{63}< 6\)
Câu hỏi của Hoàng Đỗ Việt - Toán lớp 6 | Học trực tuyến
Bài 1 :
Ta có;\(\frac{1}{21}+\frac{1}{22}+\frac{1}{23}+...+\frac{1}{30}>\frac{1}{30}.10=\frac{1}{3}\)
\(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{60}>\frac{1}{60}.30>\frac{1}{30}.24=\frac{2}{5}\)
Do đó :
\(\frac{1}{21}+\frac{1}{22}+...+\frac{1}{60}>\frac{1}{3}+\frac{2}{5}=\frac{11}{15}\left(1\right)\)
Mặt khác :
\(\frac{1}{21}+\frac{1}{22}+...+\frac{1}{40}< \frac{1}{20}.20=1\)
\(\frac{1}{41}+\frac{1}{42}+...+\frac{1}{60}< \frac{1}{40}.20=\frac{1}{2}\)
Do đó :
\(\frac{1}{21}+\frac{1}{22}+...+\frac{1}{60}< 1+\frac{1}{2}=\frac{3}{2}\left(2\right)\)
Từ (1 ) và (2) ta suy ra điều phải chứng minh
Bài 2 :
Đặt \(S=1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{63}\)
MỘT MẶT ,TA CÓ THỂ VIẾT
\(S=\left(1+\frac{1}{2}\right)+\left(\frac{1}{3}+\frac{1}{4}\right)+\left(\frac{1}{5}+\frac{1}{6}+\frac{1}{7}+\frac{1}{8}\right)\)\(+\left(\frac{1}{9}+\frac{1}{10}+...+\frac{1}{16}\right)+\left(\frac{1}{17}+\frac{1}{18}+...+\frac{1}{32}\right)\)\(+\left(\frac{1}{33}+\frac{1}{34}+...+\frac{1}{63}+\frac{1}{64}\right)-\frac{1}{64}\)
\(>\frac{1}{2}.2+\frac{1}{4}.2+\frac{1}{8}.4+\frac{1}{16}.8+\frac{1}{32}.16+\frac{1}{64}.32-\frac{1}{64}\)\(=\frac{7}{2}-\frac{1}{64}=\frac{223}{64}>\frac{192}{64}=3\left(1\right)\)
Mặt khác ,ta lại có\(S=1+\left(\frac{1}{2}+\frac{1}{3}\right)+\left(\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}\right)\)\(+\left(\frac{1}{8}+\frac{1}{9}+...+\frac{1}{15}\right)+\left(\frac{1}{16}+\frac{1}{17}+...+\frac{1}{31}\right)\)\(+\left(\frac{1}{32}+\frac{1}{33}+...+\frac{1}{63}\right)< \)\(1+\frac{1}{2}.2+\frac{1}{4}.4+\frac{1}{8}.8+\frac{1}{16}.16+\frac{1}{32}.32=6\left(2\right)\)
Từ (1) và (2 ) ta kết luận \(3< S< 6\)
Chúc bạn học tốt ( -_- )
a) Đặt \(A=\frac{1}{21}+\frac{1}{22}+\frac{1}{23}+...+\frac{1}{59}+\frac{1}{60}\)
Ta có:
\(A=\left(\frac{1}{21}+\frac{1}{22}+...+\frac{1}{40}\right)+\left(\frac{1}{41}+\frac{1}{42}+...+\frac{1}{60}\right)\)
+ Vì \(\frac{1}{21}>\frac{1}{40};\frac{1}{22}>\frac{1}{40};...;\frac{1}{40}=\frac{1}{40}\)
\(\Rightarrow\frac{1}{21}+\frac{1}{22}+...+\frac{1}{40}>\frac{1}{40}+\frac{1}{40}+...+\frac{1}{40}\)( 20 phân số \(\frac{1}{40}\)) \(=20.\frac{1}{40}=\frac{1}{2}.\)
+ Vì \(\frac{1}{41}>\frac{1}{60};\frac{1}{42}>\frac{1}{60};...;\frac{1}{60}=\frac{1}{60}\)
\(\Rightarrow\frac{1}{41}+\frac{1}{42}+...+\frac{1}{60}>\frac{1}{60}+\frac{1}{60}+...+\frac{1}{60}\)( 20 phân số \(\frac{1}{60}\)) \(=20.\frac{1}{60}=\frac{1}{3}\)
\(\Rightarrow A>\frac{1}{2}+\frac{1}{3}=\frac{5}{6}=\frac{75}{90}>\frac{66}{90}=\frac{11}{15}\)
\(\Rightarrow A>\frac{11}{15}\left(1\right)\)
Lại có:
+ Vì \(\frac{1}{21}< \frac{1}{20};\frac{1}{22}< \frac{1}{20};...;\frac{1}{40}< \frac{1}{20}\)
\(\Rightarrow\frac{1}{21}+\frac{1}{22}+...+\frac{1}{40}< \frac{1}{20}+\frac{1}{20}+...+\frac{1}{20}\)( 20 phân số \(\frac{1}{20}\)) \(=20.\frac{1}{20}=1\)
+ Vì \(\frac{1}{41}< \frac{1}{40};\frac{1}{42}< \frac{1}{40};...;\frac{1}{60}< \frac{1}{40}\)
\(\Rightarrow\frac{1}{41}+\frac{1}{42}+...+\frac{1}{60}< \frac{1}{40}+\frac{1}{40}+...+\frac{1}{40}\)( 20 phân số \(\frac{1}{40}\)) \(=20.\frac{1}{40}=\frac{1}{2}\)
\(\Rightarrow A< 1+\frac{1}{2}=\frac{3}{2}\)
\(\Rightarrow A< \frac{3}{2}\left(2\right)\)
Từ\(\left(1\right)\);\(\left(2\right)\) \(\Rightarrow\frac{11}{15}< A< \frac{3}{2}\left(đpcm\right).\)
b) Đặt \(B=1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{63}\)
Ta có:
\(B=1+\left(\frac{1}{2}+\frac{1}{3}\right)+\left(\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}\right)+\left(\frac{1}{8}+\frac{1}{9}+...+\frac{1}{15}\right)\)\(+\left(\frac{1}{16}+\frac{1}{17}+...+\frac{1}{31}\right)+\left(\frac{1}{32}+\frac{1}{33}+...+\frac{1}{63}\right)\)
+ \(1=1\)
+\(\frac{1}{2}+\frac{1}{3}< \frac{1}{2}+\frac{1}{2}=1\)
+\(\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}< \frac{1}{4}+\frac{1}{4}+\frac{1}{4}+\frac{1}{4}=1\)
+\(\frac{1}{8}+\frac{1}{9}+...+\frac{1}{15}< \frac{1}{8}+\frac{1}{8}+...+\frac{1}{8}=1\)
Tương tự ta được:
+\(\frac{1}{16}+\frac{1}{17}+...+\frac{1}{31}< 1\)
+\(\frac{1}{32}+\frac{1}{33}+...+\frac{1}{63}< 1\)
\(\Rightarrow A< 1+1+1+1+1+1=6\left(1\right)\)
Lại có:
+\(1=1\)
+\(\frac{1}{2}+\frac{1}{3}>\frac{1}{3}+\frac{1}{3}=\frac{2}{3}\)
+\(\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}>\frac{1}{7}+\frac{1}{7}+\frac{1}{7}+\frac{1}{7}=\frac{4}{7}\)
+\(\frac{1}{8}+\frac{1}{9}+...+\frac{1}{15}>\frac{1}{15}+\frac{1}{15}+...+\frac{1}{15}=\frac{8}{15}\)
Tương tự, ta được:
+\(\frac{1}{16}+\frac{1}{17}+...+\frac{1}{31}>\frac{16}{31}\)
+\(\frac{1}{32}+\frac{1}{33}+...+\frac{1}{63}< \frac{32}{63}\)
\(\Rightarrow A>1+\frac{2}{3}+\frac{4}{7}+\frac{8}{15}+\frac{16}{31}+\frac{32}{63}\)\(=1+\frac{18}{15}+\frac{64}{63}+\frac{16}{31}>1+\frac{15}{15}+\frac{63}{63}=3\left(2\right)\)
Từ\(\left(1\right)\)và \(\left(2\right)\Rightarrow3< A< 6\left(đpcm\right).\)
Cho \(A=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\)
Chứng tỏ rằng A > 1
\(A=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\)
\(A=\frac{1}{10}+\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\right)>\frac{1}{10}+\left(\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}\right)\)
\(A=\frac{1}{10}+\frac{99}{100}=1\)
=> A > 1
\(A=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\)
\(A=\frac{1}{10}+\frac{1}{11}+...+\frac{1}{19}>\frac{1}{20}+\frac{1}{20}+...+\frac{1}{20}=\frac{10}{20}=\frac{1}{2}\)
\(A=\frac{1}{20}+\frac{1}{21}+...+\frac{1}{29}>\frac{1}{30}+\frac{1}{30}+...+\frac{1}{30}=\frac{10}{30}=\frac{1}{3}\)
\(A=\frac{1}{30}+\frac{1}{31}+...+\frac{1}{39}>\frac{1}{40}+\frac{1}{40}+... +\frac{1}{40}=\frac{10}{40}=\frac{1}{4}\)
\(\Rightarrow A>1\)
Ta thấy:1/10;1/11;1/12;1/13;...;1/99>1/100
=)1/10+1/11+1/12+1/13+...+1/100>1/100+1/100+1/100+1/100..+1/100=1/100.100=1
Vậy A>1
Cho tổng A=\(\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...\frac{1}{99}+\frac{1}{100}\)
Chứng tỏ rằng A > 1
\(A=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{100}\)
\(=\frac{1}{10}+\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\right)>\frac{1}{10}+\left(\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}\right)\)
\(=\frac{1}{10}+\frac{90}{100}>1\)
\(A>1\left(đpcm\right)\)
Cho 23!+ 19! - 15! Chứng tỏ rằng B chia hết cho 11 , B chia hết cho 110, Chứng tỏ rằng 53! -51!chia hết cho 29
Thanks
Cho A=\(\frac{7}{3}+\frac{11}{3^2}+\frac{15}{3^3}+...+\frac{2019}{3^{504}}\)
Chứng tỏ A<\(\frac{9}{2}\)
Chứng tỏ rằng:
A=\(\frac{3}{10}+\frac{3}{11}+\frac{3}{12}+\frac{3}{13}+\frac{3}{14}\)
Thì 1<A<2