Phương án C.

How often do you stay in bed late?

a. \(\left\{{}\begin{matrix}4x-3y=-10\\2x+5y=8\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}4x-3y=-10\\4x+10y=16\end{matrix}\right.\)

Trừ vế cho vế:

\(\Leftrightarrow-13y=26\)

\(\Leftrightarrow y=2\)

Thay y = 2 vào 2x + 5y = 8 ta được:

\(2x+5\cdot2=8\Leftrightarrow x=-1\)

Vậy \(\left(x;y\right)=\left(-1;2\right)\)

b. \(\left\{{}\begin{matrix}5x+3y=19\\2x+9y=31\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}10x+6y=38\\10x+45y=155\end{matrix}\right.\)

Trừ vế cho vế:

\(\Leftrightarrow-39y=-117\Leftrightarrow y=3\)

Thay y = 3 vào 2x + 9y = 31 ta được:

\(2x+9\cdot3=31\Leftrightarrow x=2\)

Vậy \(\left(x;y\right)=\left(2;3\right)\)

\(\left\{{}\begin{matrix}-5x+y=10\\x+3y=18\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-15x+3y=30\\x+3y=18\end{matrix}\right.\)

Trừ vế cho vế, ta được:

\(\Leftrightarrow-16x=48\)

\(\Leftrightarrow x=-3\)

Thay \(x=-3\) vào \(x+3y=-18\) ta được:

\(-3+3y=-18\Leftrightarrow y=-5\)

Vậy \(\left(x;y\right)=\left(-3;-5\right)\)

Giải hệ phương trình 2 ẩn:

\(\left\{{}\begin{matrix}\dfrac{x+y}{3}+\dfrac{2}{3}=3\\\dfrac{4x-y}{6}+\dfrac{x}{4}=1\end{matrix}\right.\)

\(489\times85-75\times489\)

\(=489\times\left(85-75\right)\)

\(=489\times10\)

\(=4890\)

isn't working

Khi \(x=\dfrac{5}{6}\) thì:

Vì  \(\dfrac{5}{6}:3=\dfrac{5}{18}\)

nên \(\dfrac{5}{18}=\dfrac{2}{5}\) (Sai)

Vậy khẳng định trên là sai.

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

\(\Rightarrow x=4\)

\(3\times\left(2+x\right)-15=31\)

\(3\times\left(2+x\right)=31+15\)

\(3\times\left(2+x\right)=46\)

\(2+x=46:3\)

\(2+x=15,3\)

\(x=15,3-2\)

\(x=13,3\)