\(\Leftrightarrow\)\(\left[{}\begin{matrix}x=-\dfrac{1}{2}\\x=\dfrac{5}{12}\end{matrix}\right.\)

2d)

x²-2.x.\(\dfrac{1}{2}\)+\(\left(\dfrac{1}{2}\right)^2\)=0

\(\Leftrightarrow\)(x-\(\dfrac{1}{2}\))²=0

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

2c)

x(3x-2)+5(3x-2)=0

\(\Leftrightarrow\)(3x-2)(x+5)=0

\(\Leftrightarrow\)\(\left[{}\begin{matrix}3x-2=0\\x+5=0\end{matrix}\right.\)

\(\Leftrightarrow\)\(\left[{}\begin{matrix}x=\dfrac{2}{3}\\x=-5\end{matrix}\right.\)

2b)

\(\left(\dfrac{2}{3}\right)^2-\left(5x\right)^2=0\)

\(\Leftrightarrow\)(\(\dfrac{2}{3}-5x\))(\(\dfrac{2}{3}+5x\))=0

\(\Leftrightarrow\)\(\left[{}\begin{matrix}\dfrac{2}{3}-5x=0\\\dfrac{2}{3}+5x=0\end{matrix}\right.\)

\(\Leftrightarrow\)\(\left[{}\begin{matrix}x=\dfrac{2}{15}\\x=-\dfrac{2}{15}\end{matrix}\right.\)

2a)

(x+1)(x²+2x)=0

(x+1)x(x+2)=0

\(\left[{}\begin{matrix}x+1=0\\x=0\\x+2=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=0\\x=-2\end{matrix}\right.\)

a) 5x-15y=5x-3.5.y=5(x-3y)

c) 14xy(xy+28x)

d) \(\dfrac{2}{7}\left(3x-1\right)\left(x-1\right)\)

e) (x-1)³

f) (x+y-2x)(x+y+2x)=(y-x)(3x+y)

g) (3x+\(\dfrac{1}{2}\))(9x²+\(\dfrac{3}{2}x\)+\(\dfrac{1}{4}\))

h) (x+y-x+y)\(\left[\left(x+y\right)^2-\left(x+y\right)\left(x-y\right)+\left(x-y\right)^2\right]\)

\(n_{_{ }HCl}=0,25mol\)

\(n_{H_2}=0,1mol\)

A+2HCl\(\rightarrow\)ACl₂+H₂

\(n_{ACl_2}=n_{H_2}=0,1mol\)

ACl₂=\(\dfrac{20,8}{0,1}=208\)(đvC)

hay: A+71=208 suy ra A=137(Bari: Ba)

dd x(HCl dư: 0,25-0,1.2=0,05mol và BaCl₂ 0,1 mol)

HCl+AgNO₃\(\rightarrow\)AgCl+HNO₃

BaCl₂+2AgNO₃\(\rightarrow\)2AgCl+Ba(NO₃)₂

\(n_{AgCl}=n_{HCl}+2n_{BaCl_2}=0,05+2.0,1=0,25mol\)

m=0,25.143,5=35,875g

Câu 2:

- Ý 1: 1s²2s²2p⁶3s²3p³(5e: phi kim)

^{- Ý 2:} 1s²2s²2p¹(3e kim loại)

- Ý 3: 1s²2s²2p⁶3s²3p⁴( 6e lớp ngoài cùng nên là phi kim)

Coi giá bán lúc chưa giảm giá là 100% thì giá bán sau khi giảm giá là: 100 - 20 = 80 (%)

Số hàng mua được lúc sau so với lúc chưa giảm giá là :

          100 : 80 x 100 = 125 (%)

Số hàng mua được tăng thêm là:

          125 - 100 = 25 (%)

                       Đáp số 25%

nguyên tố s.......

1s²2s²2p⁶3s²3p⁶4s²