Đáp án: A

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Chỗ này nha bạn

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Câu 1: B

Câu 2: D

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Đáp án: 15

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

\(=\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{12}+\dfrac{1}{12}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{14}+\dfrac{1}{14}-\dfrac{1}{15}\)

\(=\dfrac{1}{9}-\dfrac{1}{15}\)

\(=\dfrac{2}{45}\)

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\(1-\left(5\dfrac{3}{8}+x-7\dfrac{5}{24}\right):\left(-16\dfrac{2}{3}\right)=0\)

\(\Leftrightarrow1-\left(\dfrac{43}{8}+x-\dfrac{173}{24}\right):\dfrac{50}{3}=0\)

\(\Leftrightarrow1-\left(\dfrac{129}{24}-\dfrac{173}{24}+x\right).\dfrac{3}{50}=0\)

\(\Leftrightarrow1+\dfrac{11}{6}-x.\dfrac{3}{50}=0\)

\(\Leftrightarrow\dfrac{17}{6}-x.\dfrac{3}{50}=0\)

\(\Leftrightarrow x.\dfrac{3}{50}=\dfrac{17}{6}\)

\(\Leftrightarrow x=\dfrac{425}{9}\)

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Đề 1

Bài 1:

a, \(\dfrac{2}{3}-\dfrac{5}{7}.\dfrac{14}{25}\)

\(=\dfrac{2}{3}-\dfrac{5}{15}.\dfrac{14}{7}\)

\(=\dfrac{2}{3}-\dfrac{1.2}{3}\)

\(=0\)

b, \(\dfrac{-2}{5}.\dfrac{5}{8}+\dfrac{5}{8}.\dfrac{3}{5}\)

\(=\dfrac{5}{3}.\left(-\dfrac{2}{3}+\dfrac{3}{5}\right)\)

\(=\dfrac{5}{8}.\dfrac{1}{5}=\dfrac{1}{8}\)

c, \(25\%-1\dfrac{1}{2}+0,5.\dfrac{12}{5}\)

\(=0,25-2,5+1,2\)

\(=-1,05\)

Bài 2:

a, \(x+\dfrac{1}{2}=\dfrac{3}{4}\)

\(x=\dfrac{3}{4}-\dfrac{1}{2}\)

\(x=\dfrac{1}{4}\)

b, \(\dfrac{4}{5}x=\dfrac{4}{7}\)

\(x=\dfrac{4}{7}:\dfrac{4}{5}\)

\(x=\dfrac{5}{7}\)

c, \(8x=7,8x+25\)

\(\Leftrightarrow8x-7,8=25\)

\(\Leftrightarrow0,2x=25\)

\(\Leftrightarrow x=25:0,2\)

\(\Leftrightarrow x=125\)

Đề 2

Bài 1:

a, \(\dfrac{7.9-14}{3-17}\)\(=\dfrac{63-14}{-14}=-\dfrac{7}{2}\)

b,\(0,25.2\dfrac{1}{3}30.0,5.\dfrac{8}{45}\)

\(=\dfrac{1}{4}.\dfrac{7}{3}.30.\dfrac{1}{2}.\dfrac{8}{45}\)

\(=\dfrac{1.7.15.2.1.8}{4.2.3.15.3}\)

\(=\dfrac{7.2}{3.3}\)

\(=\dfrac{14}{9}\)

c, \(\dfrac{9}{23}.\dfrac{5}{8}+\dfrac{9}{23}.\dfrac{3}{8}-\dfrac{9}{23}\)

\(=\dfrac{9}{23}.\left(\dfrac{5}{8}+\dfrac{3}{8}\right)-\dfrac{9}{23}\)

\(=\dfrac{9}{23}.1-\dfrac{9}{23}=0\)

 Bài 2:

a, \(\dfrac{1}{2}-\left(\dfrac{2}{3}x-\dfrac{1}{3}\right)=\dfrac{2}{3}\)

\(\Leftrightarrow\dfrac{2}{3}x-\dfrac{1}{3}=\dfrac{1}{2}-\dfrac{2}{3}\)

\(\Leftrightarrow\dfrac{2}{3}x-\dfrac{1}{3}=-\dfrac{1}{6}\)

\(\Leftrightarrow\dfrac{2}{3}x=-\dfrac{1}{6}+\dfrac{1}{3}\)

\(\Leftrightarrow\dfrac{2}{3}x=\dfrac{1}{6}\)

\(x=\dfrac{1}{4}\)

b, \(\dfrac{3}{x+5}=15\%\)

\(\Leftrightarrow\dfrac{3}{x+5}=\dfrac{3}{20}\)

\(\Leftrightarrow x+5=20\)

\(\Leftrightarrow x=20-5\)

\(\Rightarrow x=15\)

Bài 3:

\(A=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{49.50}\)

\(=\left(1-\dfrac{1}{2}\right)+\left(\dfrac{1}{2}-\dfrac{1}{3}\right)+\left(\dfrac{1}{3}-\dfrac{1}{4}\right)+...+\left(\dfrac{1}{49}-\dfrac{1}{50}\right)\)

\(\Rightarrow\) Do  \(-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}+...+\dfrac{1}{49}=0\)

\(\Rightarrow A=1-\dfrac{1}{50}=\dfrac{49}{50}\)

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Chiều bể nước hình hộp chữ nhật là:

8x\(\dfrac{3}{2}=12\left(cm\right)\)

Chiều cao bể nước hình hộp chữ nhật là:

\(\left(8+12\right):2=10\left(cm\right)\)

Thể thích bể là:

8x10x12\(=960\left(cm^3\right)\)

Thể tích viên đá là:

960x\(\left(1-\dfrac{7}{10}\right)=288\left(cm^3\right)\)

Đáp số: 288 (cm\(^3\))

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\(9-3:\dfrac{1}{3}+1\)

\(=9-3.3+1\)

\(=9-9+1\)

\(=1\)

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