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1: B là số nguyên
=>n-3 thuộc {1;-1;5;-5}
=>n thuộc {4;2;8;-2}
3:
a: -72/90=-4/5
b: 25*11/22*35
\(=\dfrac{25}{35}\cdot\dfrac{11}{22}=\dfrac{5}{7}\cdot\dfrac{1}{2}=\dfrac{5}{14}\)
c: \(\dfrac{6\cdot9-2\cdot17}{63\cdot3-119}=\dfrac{54-34}{189-119}=\dfrac{20}{70}=\dfrac{2}{7}\)
Bài 1:
a)
\(\dfrac{1}{2}=\dfrac{1\times6}{2\times6}=\dfrac{6}{12}\)
\(\dfrac{2}{3}=\dfrac{2\times4}{3\times4}=\dfrac{8}{12}\)
\(\dfrac{3}{4}=\dfrac{3\times3}{4\times3}=\dfrac{9}{12}\)
b)
\(\dfrac{1}{3}=\dfrac{1\times15}{3\times15}=\dfrac{15}{45}\)
\(\dfrac{2}{15}=\dfrac{2\times3}{15\times3}=\dfrac{6}{45}\)
\(\dfrac{4}{45}\) (giữ nguyên)
c)
\(\dfrac{1}{8}=\dfrac{1\times3}{8\times3}=\dfrac{3}{24}\)
\(\dfrac{2}{3}=\dfrac{2\times8}{3\times8}=\dfrac{16}{24}\)
\(\dfrac{5}{2}=\dfrac{5\times12}{2\times12}=\dfrac{60}{24}\)
d)
\(\dfrac{2}{7}=\dfrac{2\times4}{7\times4}=\dfrac{8}{28}\)
\(\dfrac{9}{4}=\dfrac{9\times7}{4\times7}=\dfrac{63}{28}\)
\(\dfrac{5}{28}\) (giữ nguyên)
Bài 2:
a)
\(4=\dfrac{4}{1}=\dfrac{4\times12}{1\times12}=\dfrac{48}{12}\)
\(\dfrac{9}{4}=\dfrac{9\times3}{4\times3}=\dfrac{27}{12}\)
b)
\(\dfrac{5}{8}=\dfrac{5\times30}{8\times30}=\dfrac{150}{240}\)
\(\dfrac{25}{30}=\dfrac{5}{6}=\dfrac{5\times40}{6\times40}=\dfrac{200}{240}\)
\(2=\dfrac{2}{1}=\dfrac{2\times240}{1\times240}=\dfrac{480}{240}\).
Bài 2:
a: -2*(-27)=54
6*9=54
=>Hai phân số này bằng nhau
b: -1/-5=1/5=5/25<>4/25
Bài 3:
a: =>16/x=-4/5
=>x=-20
b: =>(x+7)/15=-2/3
=>x+7=-10
=>x=-17
a: \(\dfrac{-7}{6}=\dfrac{-7\cdot3}{6\cdot3}=\dfrac{-21}{18}\)
\(\dfrac{-11}{9}=\dfrac{-11\cdot2}{9\cdot2}=\dfrac{-22}{18}\)
mà -21>-22
nên \(-\dfrac{7}{6}>-\dfrac{11}{9}\)
b: \(\dfrac{5}{-7}=\dfrac{-5}{7}=\dfrac{-5\cdot5}{7\cdot5}=\dfrac{-25}{35}\)
\(\dfrac{-4}{5}=\dfrac{-4\cdot7}{5\cdot7}=\dfrac{-28}{35}\)
mà -25>-28
nên \(\dfrac{5}{-7}>\dfrac{-4}{5}\)
c: \(\dfrac{-8}{7}< -1\)
\(-1< -\dfrac{2}{5}\)
Do đó: \(-\dfrac{8}{7}< -\dfrac{2}{5}\)
d: \(-\dfrac{2}{5}< 0\)
\(0< \dfrac{1}{3}\)
Do đó: \(-\dfrac{2}{5}< \dfrac{1}{3}\)
a) \(\dfrac{20}{12}\) b) \(\dfrac{15}{15}\) c) \(\dfrac{24}{42}\)
d) \(\dfrac{26}{39}\) e) \(\dfrac{0}{8}\) g) \(\dfrac{15}{3}\)
a) \(\dfrac{15}{3}\)
b)\(\dfrac{15}{15}\)
c) \(\dfrac{24}{42}\)
d) \(\dfrac{26}{39}\)
e)\(\dfrac{0}{8}\)
f) \(\dfrac{15}{15}\)
a) \(\dfrac{6}{14}=\dfrac{6:2}{14:2}=\dfrac{3}{7}\)
\(\dfrac{3}{7}< \dfrac{4}{7}\)
b) \(\dfrac{6}{15}=\dfrac{6:3}{15:3}=\dfrac{2}{5}\)
\(\dfrac{3}{5}>\dfrac{2}{5}\)
c) \(\dfrac{10}{18}=\dfrac{10:2}{18:2}=\dfrac{5}{9}\)
\(\dfrac{5}{9}>\dfrac{2}{9}\)
2/
a/ \(\dfrac{7}{10}=\dfrac{7.15}{10.15}=\dfrac{105}{150}\)
\(\dfrac{11}{15}=\dfrac{11.10}{15.10}=\dfrac{110}{150}\)
-Vì \(\dfrac{105}{150}< \dfrac{110}{150}\)(105<110)nên \(\dfrac{7}{10}< \dfrac{11}{15}\)
b/ \(\dfrac{-1}{8}=\dfrac{-1.3}{8.3}=\dfrac{-3}{24}\)
-Vì \(\dfrac{-3}{24}>\dfrac{-5}{24}\left(-3>-5\right)\)nên\(\dfrac{-1}{8}>\dfrac{-5}{24}\)
c/\(\dfrac{25}{100}=\dfrac{25:25}{100:25}=\dfrac{1}{4}\)
\(\dfrac{10}{40}=\dfrac{10:10}{40:10}=\dfrac{1}{4}\)
-Vì \(\dfrac{1}{4}=\dfrac{1}{4}\)nên\(\dfrac{25}{100}=\dfrac{10}{40}\)
a/ \(\dfrac{7}{10}< \dfrac{11}{15}\)
c/ \(\dfrac{25}{100}=\dfrac{10}{40}\)
a. Ta có : \(\dfrac{{ - 3}}{5} = \dfrac{9}{{ - 15}}\) vì (-3).(-15)=5.9 (=45)
b. Ta có : \(\dfrac{{ - 1}}{{ - 4}} = \dfrac{1}{4}\) vì (-1).4=(-4).1 (=-4)
Hai phân số bằng nhau là \(\dfrac{15}{-10}\) và \(-\dfrac{3}{2}\)
Bài 1: So sánh các cặp phân số sau:
a) \(\dfrac{4}{15}\) và \(\dfrac{3}{5}\)
\(\Rightarrow\dfrac{4}{15}=\dfrac{4\times5}{15\times5}=\dfrac{20}{75}\)
\(\Rightarrow\dfrac{3}{5}=\dfrac{3\times15}{5\times15}=\dfrac{45}{75}\)
Vì: \(20< 45\) Nên \(\dfrac{4}{15}\) < \(\dfrac{3}{5}\)
b) \(\dfrac{2}{3}\) và \(\dfrac{4}{7}\)
\(\Rightarrow\dfrac{2}{3}=\dfrac{2\times7}{3\times7}=\dfrac{14}{21}\)
\(\Rightarrow\dfrac{4}{7}=\dfrac{4\times3}{7\times3}=\dfrac{12}{21}\)
Vì: \(14>12\) Nên: \(\dfrac{2}{3}>\dfrac{4}{7}\)
c) \(\dfrac{15}{12}\) và \(\dfrac{20}{28}\)
\(\Rightarrow\dfrac{15}{12}=\dfrac{15\times28}{12\times28}=\dfrac{420}{336}\)
\(\Rightarrow\dfrac{20}{28}=\dfrac{20\times12}{28\times12}=\dfrac{240}{336}\)
Vì: \(420>240\) Nên: \(\dfrac{15}{12}>\dfrac{20}{28}\)
a) \(\dfrac{4}{15}< \dfrac{3}{5}\)
Vì khi quy đồng \(\dfrac{3}{5}=\dfrac{3x3}{5x3}=\dfrac{9}{15}\)
Vì \(\dfrac{4}{15}< \dfrac{9}{15}\) nên \(\dfrac{4}{15}< \dfrac{3}{5}\)
b) \(\dfrac{2}{3}>\dfrac{4}{7}\)
Vì khi quy đồng \(\dfrac{2}{3}=\dfrac{2x7}{3x7}=\dfrac{14}{21}\); \(\dfrac{4}{7}=\dfrac{4x3}{7x3}=\dfrac{12}{21}\) nên \(\dfrac{2}{3}>\dfrac{4}{7}\)
c) \(\dfrac{15}{12}>\dfrac{20}{28}\)
Vì khi quy đồng \(\dfrac{15}{12}=\dfrac{15x28}{12x28}=\dfrac{420}{336}\) ; \(\dfrac{20}{28}=\dfrac{20x12}{28x12}=\dfrac{240}{336}\) nên \(\dfrac{15}{12}>\dfrac{20}{28}\)