a,PTK là 35,5.2.2=142 (đvC)

b,Ta có: 2.M_X + 5.16=142

        <=> 2M_X = 62

        <=> M_X = 31

=> X là photpho (P) 

5.C

6.C

7.D

Ta có:\(A=3x+2y+\dfrac{6}{x}+\dfrac{8}{y}\)

       \(\Leftrightarrow2A=6x+4y+\dfrac{12}{x}+\dfrac{16}{y}=\left(3x+\dfrac{12}{x}\right)+\left(y+\dfrac{16}{y}\right)+3\left(x+y\right)\)

Áp dụng bất đẳng thức Cô-si ta có:

\(3x+\dfrac{12}{x}\ge2\sqrt{3x.\dfrac{12}{x}}=12;y+\dfrac{16}{y}\ge2\sqrt{y.\dfrac{16}{y}}=8\)

Ta có:3(x+y)≥3.6=18

Cộng vế với vế ta có: 2A≥38⇔A≥19

Dấu "=" xảy ra ⇔ x=2;y=4

Vậy Min A=19⇔ x=2;y=4

có lẽ 8/y

6.spare ,nhìn lộn thành space

1.in

2.favourite

3.join

4.spends

5.hobby

6.empty

7.member

8.the cinema

Ta có:\(A=\left(\dfrac{a+4\sqrt{a}+4}{a+2\sqrt{a}}-\dfrac{\sqrt{a}}{\sqrt{a}-2}\right):\left(\dfrac{\sqrt{a}-4}{a-2\sqrt{a}}-\dfrac{3\sqrt{a}+6}{4-a}\right)\)

             \(=\left[\dfrac{\left(\sqrt{a}+2\right)^2}{\sqrt{a}\left(\sqrt{a}+2\right)}-\dfrac{\sqrt{a}}{\sqrt{a}-2}\right]:\left[\dfrac{\sqrt{a}-4}{\sqrt{a}\left(\sqrt{a}-2\right)}+\dfrac{3\left(\sqrt{a}+2\right)}{\left(\sqrt{a}+2\right)\left(\sqrt{a}-2\right)}\right]\)

             \(=\dfrac{a-4-a-2\sqrt{a}}{\sqrt{a}\left(\sqrt{a}-2\right)}:\dfrac{\sqrt{a}-4+3\sqrt{a}}{\sqrt{a}\left(\sqrt{a}-2\right)}\)

            \(=\dfrac{-4-2\sqrt{a}}{\sqrt{a}\left(\sqrt{a}-2\right)}.\dfrac{\sqrt{a}\left(\sqrt{a}-2\right)}{4\sqrt{a}-4}=\dfrac{-2-\sqrt{a}}{2\sqrt{a}-2}\)

1.7 km² 2 ha =7,02 km²

2.4/5ha = 8000 m²

A.chuồn chuồn nước

27.interesting

28.editor

29.available

30.education

31.dangerous

32.cloudy

33.destruction

34.differs