Tính:B=6/8+6/56+6/140+...+6/1100+6/1400
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NH
3
4 tháng 5 2021
ta có A=\(\dfrac{6}{8}\)+\(\dfrac{6}{56}\)+\(\dfrac{6}{140}\)+...+\(\dfrac{6}{1100}\)+\(\dfrac{6}{1400}\)
=\(\dfrac{3}{4}\)+\(\dfrac{3}{28}\)+\(\dfrac{3}{70}\)+...+\(\dfrac{3}{550}\)+\(\dfrac{3}{700}\)
=\(\dfrac{3}{1.4}\)+\(\dfrac{3}{4.7}\)+\(\dfrac{3}{7.10}\)+...+\(\dfrac{3}{22.25}\)+\(\dfrac{3}{25.28}\)
=1-\(\dfrac{1}{4}\)+\(\dfrac{1}{4}\)-\(\dfrac{1}{7}\)+\(\dfrac{1}{7}\)-\(\dfrac{1}{10}\)+...+\(\dfrac{1}{22}\)-\(\dfrac{1}{25}\)+\(\dfrac{1}{25}\)-\(\dfrac{1}{28}\)
=1-\(\dfrac{1}{28}\)
=\(\dfrac{27}{28}\)
Vậy A=\(\dfrac{27}{28}\)
YN
4 tháng 5 2021
Ta có:
A =6/8+6/56+6/140+...+6/1100+6/1400
⇒A=3/4+3/28+3/70+...+3/550+3/700
⇒A=3/1.4+3/4.7+3/7.10+...+3/22.25+3/25.28
⇒A=1−1/4+1/4−1/7+1/7−1/10+...+1/22−1/25+1/25−1/28
⇒A=1−1/28
⇒A=1-1/38
HH
11 tháng 4 2017
\(A=\dfrac{3}{4}+\dfrac{3}{28}+\dfrac{3}{140}+...+\dfrac{3}{550}+\dfrac{3}{700}\)
\(A=\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.10}+...+\dfrac{3}{22.25}+\dfrac{3}{25.28}\)
\(A=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{22}-\dfrac{1}{25}+\dfrac{1}{25}-\dfrac{1}{28}\)
\(A=1-\dfrac{1}{28}\)
\(A=\dfrac{28}{28}-\dfrac{1}{28}=\dfrac{27}{28}\)
TM
1
LL
1
14 tháng 3 2016
\(\frac{5}{6}+6\frac{1}{6}.\left(11\frac{94}{1591}-6\frac{38}{1951}\right):8\frac{11}{43}\)
= \(\frac{5}{6}+\frac{37}{6}.\frac{8011}{1591}:8\frac{11}{43}\)
= \(\frac{5}{6}+\frac{8011}{258}:\frac{355}{43}\)
= \(\frac{5}{6}+\frac{344473}{91590}\)
= \(\frac{1631}{355}\)
20 tháng 8 2021
\(1,\)
\(2x\left(x-3\right)-\left(3-x\right)=0\)
\(\Leftrightarrow2x\left(x-3\right)+\left(x-3\right)=0\)
\(\Leftrightarrow\left(2x+1\right)\left(x-3\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}2x+1=0\\x-3=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=\frac{-1}{2}\\x=3\end{cases}}\)
\(2,\)
\(3x\left(x+5\right)-6\left(x+5\right)=0\)
\(\Leftrightarrow\left(3x-6\right)\left(x+5\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}3x-6=0\\x+5=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=2\\x=-5\end{cases}}\)
\(3,\)
\(x^4-x^2=0\)
\(\Leftrightarrow x^2\left(x^2-1\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x^2=0\\x^2-1=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=0\\x=\pm1\end{cases}}\)
\(4,\)
\(x^2-2x=0\)
\(\Leftrightarrow x\left(x-2\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x=0\\x-2=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}x=0\\x=2\end{cases}}\)
\(5,\)
\(x\left(x+6\right)-10\left(x-6\right)=0\)
\(\Leftrightarrow x^2+6x-10x+60=0\)
\(\Leftrightarrow x^2-4x+60=0\)
\(\Leftrightarrow x^2-4x+4+56=0\)
\(\Leftrightarrow\left(x-2\right)^2=-56\)(Vô lý)
=> Phương trình vô nghiệm
TH
1
VT
9 tháng 3 2022
6. B
7. D
8. C
9. A
10. A
11. A
12. A
13. A
14. B
15. C
16. B
17. C
18. A
19. C
Ta có:
\(B=\frac{6}{8}+\frac{6}{56}+\frac{6}{140}+...+\frac{6}{1100}+\frac{6}{1400}\)
\(\Rightarrow B=\frac{3}{4}+\frac{3}{28}+\frac{3}{140}+...+\frac{3}{550}+\frac{3}{700}\)
\(\Rightarrow B=\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{22.25}+\frac{3}{25.28}\)
\(\Rightarrow B=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{22}-\frac{1}{25}+\frac{1}{25}-\frac{1}{28}\)
\(\Rightarrow B=1-\frac{1}{28}\)
\(\Rightarrow B=\frac{28}{28}-\frac{1}{28}=\frac{27}{28}\)
NHỚ TK MK NHA,MK ĐANG ÂM ĐIỂM
\(B=\frac{6}{8}+\frac{6}{56}+\frac{6}{140}+....+\frac{6}{1100}+\frac{6}{1400}\)
Rút gọn các phân số số ; ta được :
\(B=\frac{3}{4}+\frac{3}{56}+\frac{3}{70}+....+\frac{3}{550}+\frac{3}{700}\)
\(B=\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{22.25}+\frac{3}{25.28}\)
\(B=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{22}-\frac{1}{25}+\frac{1}{25}-\frac{1}{28}\)
\(B=1-\frac{1}{28}=\frac{27}{28}\)
Vậy biểu thức \(B=\frac{27}{28}\)