\(a,\dfrac{x}{9}=\dfrac{5}{3}\\ \Leftrightarrow x=9\cdot\dfrac{5}{3}\\ \Leftrightarrow x=15\\ b,\dfrac{17}{x}=\dfrac{85}{105}\\ \Leftrightarrow x=17\cdot\dfrac{105}{85}\\ \Leftrightarrow x=21\\ c,\dfrac{x}{8}+\dfrac{2}{3}=\dfrac{7}{6}\\ \Leftrightarrow\dfrac{x}{8}=\dfrac{1}{2}\\ \Leftrightarrow x=4\\ d,\dfrac{3}{x-7}=\dfrac{27}{135}\\ \Leftrightarrow x-7=15\\ \Leftrightarrow x=22\)

\(e,\dfrac{75}{20-x}=\dfrac{3}{2}\times10\\ \Leftrightarrow\dfrac{75}{20-x}=15\\ \Leftrightarrow20-x=5\\ \Leftrightarrow x=15\\ f,\left(x-50\%\right)\times\dfrac{5}{3}=\dfrac{7}{4}-0,5\\ \Leftrightarrow\left(x-\dfrac{1}{2}\right)\times\dfrac{5}{3}=\dfrac{5}{4}\\ \Leftrightarrow x-\dfrac{1}{2}=\dfrac{3}{4}\\ \Leftrightarrow x=\dfrac{5}{4}\\ g,\left(\dfrac{2}{15}+\dfrac{3}{35}+\dfrac{2}{63}\right):x=\dfrac{1}{18}\\ \Leftrightarrow\dfrac{2}{9}:x=\dfrac{1}{18}\\ \Leftrightarrow x=4\)

\(h,\left[\left(x-\dfrac{1}{2}\right):6+4\right]\times\dfrac{2}{3}=0,6\times\dfrac{40}{6}\\ \Leftrightarrow\left[\left(x-\dfrac{1}{2}\right):6+4\right]\times\dfrac{2}{3}=4\\ \Leftrightarrow\left(x-\dfrac{1}{2}\right):6+4=6\\ \Leftrightarrow\left(x-\dfrac{1}{2}\right):6=2\\ \Leftrightarrow x-\dfrac{1}{2}=12\\ \Leftrightarrow x=\dfrac{25}{2}\)

Bạn thử xem lại đề xem, nó không song song đâu.

\(\overline{2003ab}\) : 2;5 dư 1 ⇔ b = 1

\(\overline{2003ab}\) : 9 dư 1 ⇔ 2+0+0+3+a+b - 1⋮ 9

                              4 + a + 1 ⋮ 9

                                    5 + a ⋮ 9 ⇒ a =4; 

Thay a = 4; b = 1 vào biểu thức \(\overline{2003ab}\) ta có

\(\overline{2003ab}\) = 200341

là số 200341 nha bn

Bài 4: Thực hiện phép tính:

a, \(\dfrac{3}{4}\) \(\times\) \(\dfrac{7}{9}\) + 0,25 \(\times\) \(\dfrac{7}{9}\) - \(\dfrac{7}{9}\)

= \(\dfrac{7}{9}\) (0,75 + 0,25 - 1)

= 0 

b, \(\dfrac{5}{12}\) - \(\dfrac{1}{12}\): (\(\dfrac{3}{8}\) + \(\dfrac{1}{2}\).[\(\dfrac{9}{4}\) - 2.(19,5 -18,5)])

= \(\dfrac{5}{12}\) - \(\dfrac{1}{12}\): ( \(\dfrac{3}{8}\) + \(\dfrac{1}{2}\). [ \(\dfrac{9}{4}\)- 2])

=  \(\dfrac{5}{12}\) - \(\dfrac{1}{12}\): ( \(\dfrac{3}{8}\) + \(\dfrac{9}{8}\)- 1)

= \(\dfrac{5}{12}\) - \(\dfrac{1}{12}\): ( \(\dfrac{1}{2}\))

= \(\dfrac{5}{12}\) - \(\dfrac{1}{6}\)

= \(\dfrac{1}{4}\)

c, [ 50,2 - 0,2 .(12,05 - 10,05)] - 37,38

= [ 50,2 - 0,2. 2 ] - 37,38

= [ 50,2 - 0,4] - 37,38

= 49,8 - 37,38

= 12,42 

Bài 3: 

B = \(\dfrac{1}{1.4}\)+\(\dfrac{1}{4.7}+\dfrac{1}{7.11}+...+\dfrac{1}{97.100}\)

B = \(\dfrac{1}{3}\).( \(\dfrac{3}{1.4}\)+ \(\dfrac{3}{4.7}\)+ \(\dfrac{3}{7.11}\)+...+ \(\dfrac{3}{97.100}\))

B = \(\dfrac{1}{3}\).( \(\dfrac{1}{1}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{7}\)+ \(\dfrac{1}{7}\) - \(\dfrac{1}{11}\) +...+ \(\dfrac{1}{97}\) - \(\dfrac{1}{100}\))

B = \(\dfrac{1}{3}\).( \(\dfrac{1}{1}\) - \(\dfrac{1}{100}\))

B = \(\dfrac{1}{3}\). \(\dfrac{99}{100}\)

B = \(\dfrac{33}{100}\)

\(\dfrac{1}{5}\sqrt[]{25x+50}-5\sqrt[]{x+2}+\sqrt[]{9x+18}+9=0\)

\(\Leftrightarrow\dfrac{1}{5}\sqrt[]{25\left(x+2\right)}-5\sqrt[]{x+2}+\sqrt[]{9\left(x+2\right)}+9=0\)

\(\Leftrightarrow\dfrac{1}{5}.5\sqrt[]{x+2}-5\sqrt[]{x+2}+3\sqrt[]{x+2}+9=0\)

\(\Leftrightarrow\sqrt[]{x+2}-5\sqrt[]{x+2}+3\sqrt[]{x+2}+9=0\)

\(\Leftrightarrow\sqrt[]{x+2}\left(1-5+3\right)+9=0\)

\(\Leftrightarrow-\sqrt[]{x+2}+9=0\)

\(\Leftrightarrow\sqrt[]{x+2}=9\)

\(\Leftrightarrow x+2=81\)

\(\Leftrightarrow x=79\)

\(625,7m^2=6,257hm^2\)

0,06257

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