x = { -b \pm \sqrt { b^2-4ac } \over 2a }
\left\{\begin{array}{l}2x-y=3\\x+2y=4\end{array}\right.\Rightarrow\left\{\begin{array}{l}4x-2y=6\\x+2y=4\end{array}\right.
\Rightarrow 5x=10 \Rightarrow {x=2}
\Rightarrow 2+2y=4 \Rightarrow 2y=2 \Rightarrow y=1
A=\{\forall x\in\mathbb{N}| 10 \leqslant x \leqslant 20\}
B=\phi
\Rightarrow B \subset A
\overrightarrow{a}=\left (\begin{array}{c}x\\y\end{array}\right )=\left (\begin{array}{c}1\\2\end{array}\right )+t\left (\begin{array}{c}4\\3\end{array}\right )
\left\{\begin{array}{l}\overline{AB}\|\overline{CD}\\\overline{AC}\nparallel\overline{AB}\\\hat{C}=30^\circ\end{array}\right.\Rightarrow\hat{A_1}=30^\circ
Pb^{2+}+2Cl^-\longrightarrow{PbCl_2}\downarrow