3.

\(D=2\left(x^2+4\right)+\left(y^2+1\right)+\dfrac{28}{x}+\dfrac{1}{y}-9\)

\(D\ge8x+2y+\dfrac{28}{x}+\dfrac{1}{y}-9\)

\(D\ge7\left(x+\dfrac{4}{x}\right)+\left(y+\dfrac{1}{y}\right)+x+y-9\)

\(D\ge14\sqrt{\dfrac{4x}{4}}+2\sqrt{\dfrac{y}{y}}+3-9=24\)

\(D_{min}=24\) khi \(\left(x;y\right)=\left(2;1\right)\)

5.

\(P=12\left(x+\dfrac{4}{x}\right)+24\left(y+\dfrac{9}{y}\right)+16\left(z+\dfrac{16}{z}\right)-\left(6x+4y+3z\right)\)

\(P\ge24\sqrt{\dfrac{4x}{x}}+48\sqrt{\dfrac{9y}{y}}+32\sqrt{\dfrac{16z}{z}}-36\)

\(P\ge444\)

\(P_{min}=444\) khi \(\left(x;y;z\right)=\left(2;3;4\right)\)

Bài 4 và 6 trùng nhau?

\(A=\left(\dfrac{3x}{4}+\dfrac{3}{x}\right)+\left(\dfrac{y}{2}+\dfrac{9}{2y}\right)+\left(\dfrac{z}{4}+\dfrac{4}{z}\right)+\dfrac{1}{4}\left(x+2y+3z\right)\)

\(A\ge2\sqrt{\dfrac{9x}{4x}}+2\sqrt{\dfrac{9y}{4y}}+2\sqrt{\dfrac{4z}{4z}}+\dfrac{1}{4}.20\)

\(A\ge13\)

\(A_{min}=13\) khi \(\left(x;y;z\right)=\left(2;3;4\right)\)

lasts two months 

absent from school last Monday

prevented Mr Hoang from going to the party 

need to finish the work today

better not eat too much meat

more noise in the country than there is in the city

é tui thấy tờ giấy hình như you vẽ truyện đằng sau >:D

số đó là 60

mik nhầm nha

có phải là 840 ko bạn

Câu 3: 

a) Ta có: \(x^2-1=3\)

\(\Leftrightarrow x^2=4\)

hay \(x\in\left\{2;-2\right\}\)

b) Ta có: \(\sqrt{16x}-2\sqrt{36x}+\sqrt{9x}=2\)

\(\Leftrightarrow4\sqrt{x}-12\sqrt{x}+3\sqrt{x}=2\)

\(\Leftrightarrow-5\sqrt{x}=2\)(Vô lý)