a) \(\sqrt{9}=3\)

b) \(-\sqrt{36}=-6\)

c) \(\sqrt{\dfrac{25}{4}}=\dfrac{5}{2}\)

d) \(\sqrt{6^2}=6\)

e) \(\sqrt{\left(-6\right)^2}=\sqrt{6^2}=6\)

D

\(\dfrac{I_1}{I_2}=\dfrac{U_1}{U_2}\Rightarrow I_2=\dfrac{I_1.U_2}{U_1}=\dfrac{1,5.28}{14}=3\left(A\right)\)

Chọn D

Gọi số HS khối 6 là n(HS)(n∈N*,300<n<400)

Theo đề bài ta có: \(\left\{{}\begin{matrix}\left(n-7\right)⋮12\\\left(n-7\right)⋮15\\\left(n-7\right)⋮18\end{matrix}\right.\)

\(\Rightarrow\left(n-7\right)\in BC\left(12;15;18\right)=\left\{180;360;540;...\right\}\)

Kết hợp với ĐK

\(\Rightarrow n=367\)

a) \(4\left(n-1\right)-3⋮\left(n-1\right)\)

\(\Rightarrow\left(n-1\right)\inƯ\left(3\right)=\left\{-3;-1;1;3\right\}\)

Do \(n\in N\Rightarrow n\in\left\{0;2;4\right\}\)

b) \(-5\left(4-n\right)+12⋮\left(4-n\right)\)

\(\Rightarrow\left(4-n\right)\inƯ\left(12\right)=\left\{-12;-6;-4;-3;-2;-1;1;2;3;4;6;12\right\}\)

Do \(n\in N\Rightarrow n\in\left\{16;10;8;7;6;5;3;2;1;0\right\}\)

c) \(-2\left(n-2\right)+6⋮\left(n-2\right)\)

\(\Rightarrow\left(n-2\right)\inƯ\left(6\right)=\left\{-6;-3;-2;-1;1;2;3;6\right\}\)

Do \(n\in N\Rightarrow n\in\left\{0;1;3;4;5;8\right\}\)

d) \(n\left(n+3\right)+6⋮\left(n+3\right)\)

\(\Rightarrow\left(n+3\right)\inƯ\left(6\right)=\left\{-6;-3;-2;-1;1;2;3;6\right\}\)

Do \(n\in N\Rightarrow n\in\left\{0;3\right\}\)

\(x+2-\sqrt{x+2}-4=0\)

\(\Leftrightarrow t^2-t-4=0\Leftrightarrow\left(t-\dfrac{1}{2}\right)^2-\dfrac{17}{4}=0\)

\(\Leftrightarrow\left(t-\dfrac{1+\sqrt{17}}{2}\right)\left(t-\dfrac{1-\sqrt{17}}{2}\right)=0\)

\(\Leftrightarrow t=\dfrac{1+\sqrt{17}}{2}\left(do.t\ge0\right)\)

Chiều rộng thử ruộng hình chữ nhật: \(80:2=40\left(m\right)\)

Diện tích thửa ruộng: \(80\times40=3200\left(m^2\right)\)

\(A=\dfrac{1}{x}+\dfrac{1}{y}=\dfrac{x+y}{xy}=\dfrac{1}{0}\)(vô lý do mẫu luôn khác 0)

Vậy A k xác định với mọi x,y