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Tối 9h30 mình chốt!!!
\(\dfrac{3x}{2.5}+\dfrac{3x}{5.8}+\dfrac{3x}{8.11}+\dfrac{3x}{11.14}=\dfrac{1}{21}\)
\(\Rightarrow x\left(\dfrac{3}{2.5}+\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}\right)=\dfrac{1}{21}\)
\(\Rightarrow x\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)
\(\Rightarrow x\left(\dfrac{1}{2}-\dfrac{1}{14}\right)=\dfrac{1}{21}\)
\(\Rightarrow x.\dfrac{3}{7}=\dfrac{1}{21}\)
\(\Rightarrow x=\dfrac{1}{21}.\dfrac{7}{3}\)
\(\Rightarrow x=\dfrac{1}{9}\)
Vậy \(x=\dfrac{1}{9}\)
b)\(\dfrac{1}{7}B=\dfrac{1}{10.18}+\dfrac{1}{18.26}+\dfrac{1}{26.34}+...+\dfrac{1}{802.810}\)
\(\dfrac{1}{7}B=\dfrac{1}{8}\left(\dfrac{8}{10.18}+\dfrac{8}{18.26}+\dfrac{8}{26.34}+...+\dfrac{8}{802.810}\right)\)
\(\dfrac{1}{7}B=\dfrac{1}{8}\left(\dfrac{1}{10}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{34}+...+\dfrac{1}{802}-\dfrac{1}{810}\right)\)
\(\dfrac{1}{7}B=\dfrac{1}{8}\left(\dfrac{1}{10}-\dfrac{1}{810}\right)\)
\(\dfrac{1}{7}B=\dfrac{1}{8}.\dfrac{8}{81}\)
\(\dfrac{1}{7}B=\dfrac{1.8}{8.81}\)
\(\dfrac{1}{7}B=\dfrac{1}{81}\)
\(B=\dfrac{1}{81}:\dfrac{1}{7}\)
\(B=\dfrac{7}{81}\)
3/ Chu vi hình chữ nhật:
\(\left(\dfrac{1}{4}+\dfrac{3}{10}\right)\cdot2=\dfrac{11}{10}\) (chưa biết đơn vị)
Diện tích hình chữ nhật:
\(\dfrac{1}{4}\cdot\dfrac{3}{10}=\dfrac{11}{20}\) (chưa biết đơn vị)
\(A=\dfrac{1}{1^2}+\dfrac{1}{2^2}+\dfrac{1}{3^2}+...+\dfrac{1}{50^2}\\ \Rightarrow2A=2+\dfrac{1}{2}+\dfrac{1}{3}+...+\dfrac{1}{50}\\ \Rightarrow2A-A=2-\dfrac{1}{50^2}\\ A=\dfrac{5000}{50^2}-\dfrac{1}{50^2}=\dfrac{5000-1}{50^2}=\dfrac{4999}{2500}\)
Giả sử \(B=\dfrac{173}{100}=\dfrac{4325}{2500}\), mà \(\dfrac{4999}{2500}>\dfrac{4325}{2500}\)
\(\Rightarrow A>B\)
a) Ta có :
\(x:4\dfrac{1}{3}=-2,5\)
\(x=-2,5\times4\dfrac{1}{3}\)
\(x=-\dfrac{65}{6}\)
b) Ta có :
\(x:\dfrac{-3}{5}=\dfrac{-10}{21}\)
\(x=\dfrac{-10}{21}\times\dfrac{-3}{5}\)
\(x=\dfrac{2}{7}\)
c)\(\dfrac{2}{3}x-\dfrac{1}{2}=\dfrac{1}{10}\)
\(\dfrac{2}{3}x=\dfrac{1}{10}+\dfrac{1}{2}\)
\(\dfrac{2}{3}x=\dfrac{3}{5}\)
\(x=\dfrac{3}{5}:\dfrac{2}{3}\)
\(x=\dfrac{3}{5}\)
d) \(\dfrac{1}{2}x+\dfrac{1}{2}=\dfrac{5}{2}\)
\(\dfrac{1}{2}x=2\)
\(x=2:\dfrac{1}{2}\)
\(x=4\)
~ Chúc bn học tốt ~
a, \(x:4\dfrac{1}{3}=-2,5.\)
\(x:\dfrac{13}{3}=\dfrac{-5}{2}.\)
\(x=\dfrac{-5}{2}.\dfrac{13}{3}.\)
\(x=\dfrac{-65}{6}.\)
b, \(x:\dfrac{-3}{5}=\dfrac{-10}{21}.\)
\(x=\dfrac{-10}{21}.\dfrac{-3}{5}.\)
\(x=\dfrac{30}{105}=\dfrac{2}{7}.\)
c, \(\dfrac{2}{3}x-\dfrac{1}{2}=\dfrac{1}{10}.\)
\(\dfrac{2}{3}x=\dfrac{1}{10}+\dfrac{1}{2}.\)
\(\dfrac{2}{3}x=\dfrac{1}{10}+\dfrac{5}{10}.\)
\(\dfrac{2}{3}x=\dfrac{3}{5}.\)
\(x=\dfrac{3}{5}:\dfrac{2}{3}.\)
\(x=\dfrac{3}{5}.\dfrac{3}{2}.\)
\(x=\dfrac{9}{10}.\)
d, \(\dfrac{1}{2}x+\dfrac{1}{2}=\dfrac{5}{2}.\)
\(\dfrac{1}{2}x=\dfrac{5}{2}-\dfrac{1}{2}.\)
\(\dfrac{1}{2}x=2.\)
\(x=2:\dfrac{1}{2}.\)
\(x=2.2.\)
\(x=4.\)
~ Chúc bn học tốt!!! ~
Bài mik đúng thì nhớ tick mik nha!!!
a) Để phân số \(\dfrac{12}{n}\) có giá trị nguyên thì :
\(12⋮n\)
\(\Leftrightarrow n\inƯ\left(12\right)\)
\(\Leftrightarrow n\in\left\{-1;1;-12;12;-2;2;-6;6;-3;3;-4;4\right\}\)
Vậy \(n\in\left\{-1;1;-12;12;-2;2-6;6;-3;3;-4;4\right\}\) là giá trị cần tìm
b) Để phân số \(\dfrac{15}{n-2}\) có giá trị nguyên thì :
\(15⋮n-2\)
\(\Leftrightarrow x-2\inƯ\left(15\right)\)
Tới đây tự lập bảng zồi làm típ!
c) Để phân số \(\dfrac{8}{n+1}\) có giá trị nguyên thì :
\(8⋮n+1\)
\(\Leftrightarrow n+1\inƯ\left(8\right)\)
Lập bảng rồi làm nhs!