Gọi số học sinh nữ lớp 5A là K

Ta có \(K=30.60\%\Rightarrow K=30.\dfrac{3}{5}\Rightarrow K=6.3\Rightarrow K=18\)

Vậy số học sinh nữ lớp 5A là 18 học sinh

Ta có :

\(A=\dfrac{\dfrac{2008}{1}+\dfrac{2007}{2}+....................+\dfrac{2}{2007}+\dfrac{1}{2008}}{\dfrac{1}{2}+\dfrac{1}{3}+....................+\dfrac{1}{2008}+\dfrac{1}{2009}}\)

\(\Rightarrow A=\dfrac{\left(\dfrac{2007}{2}+1\right)+.....+\left(\dfrac{2}{2007}+1\right)+\left(\dfrac{1}{2008}+1\right)+1}{\dfrac{1}{2}+\dfrac{1}{3}+...............+\dfrac{1}{2008}+\dfrac{1}{2009}}\)

\(\Rightarrow A=\dfrac{\dfrac{2009}{2}+...................+\dfrac{2009}{2007}+\dfrac{2009}{2008}+\dfrac{2009}{2009}}{\dfrac{1}{2}+\dfrac{1}{3}+.....................+\dfrac{1}{2008}+\dfrac{1}{2009}}\)

\(\Rightarrow A=\dfrac{2009\left(\dfrac{1}{2}+..........................+\dfrac{1}{2008}+\dfrac{1}{2009}\right)}{\dfrac{1}{2}+\dfrac{1}{3}+............................+\dfrac{1}{2008}+\dfrac{1}{2009}}\)

\(\Rightarrow A=2009\)

Áp dụng tính chất \(\dfrac{a}{b}=\dfrac{c}{d}\Rightarrow ad=bc\)

a)\(\dfrac{x}{15}=\dfrac{7}{5}\Rightarrow5x=15.7\Rightarrow5x=105\Rightarrow5x=5.21\Rightarrow x=21\)

b)\(\dfrac{14}{x}=\dfrac{7}{5}\Rightarrow7x=14.5\Rightarrow7x=70\Rightarrow7x=7.10\Rightarrow x=10\)

c)\(\dfrac{2}{3}:\dfrac{1}{3}x=\dfrac{7}{5}:\dfrac{4}{5}\Rightarrow\dfrac{2}{3}.\dfrac{3}{x}=\dfrac{7}{5}.\dfrac{5}{4}\Rightarrow\dfrac{2}{x}=\dfrac{7}{4}\Rightarrow7x=2.4\Rightarrow7x=8\Rightarrow x=\dfrac{8}{7}\)

Dùng Ctrl + Z

Đặt \(\left|x-2015\right|+\left|2016-x\right|\)

Nếu \(x<2015\) thì:

\(A=2015-x+x-2016=4031-2x\)

Do \(x<2015\Rightarrow-2x>-4032\)

Nếu \(x>2016\) thì 

\(A=x-2015+x-2016=2x-4031\)

Do \(x>2016\Leftrightarrow2x>4032n\text{ê}n\text{A}>1\)

Nếu \(2015\le x\le2016\) thì

\(A=x-2015+2016-x=1\)

Vậy: \(A=\left|x-2015\right|+\left|2016-x\right|\ge1\)

d

1d

2c

3a

4b