下列四个方程组中，
①$\left\{\begin{array}{}x^{2}+3 y=4 \\ 3 x-5 y=1\end{array}\right.$②$\left\{\begin{array}{l}x y=1 \\ x+2 y=8\end{array}\right.$③$\left\{\begin{array}{l}a-b=3 \\ \frac{1}{a}-3 b=4\end{array}\right.$④$\left\{\begin{array}{l}a+3 b=4 \\ 7 a-9 b=5\end{array}\right.$二元一次方程组有个.
根据二元一次方程组的定义进行解答.
①$\left\{\begin{array}{l}x^{2}+3 y=4 \\ 3 x-5 y=1\end{array}\right.$
属于二元二次方程组.
②$\left\{\begin{array}{l}x y=1 \\ x+2 y=8\end{array}\right.$属于二元二次方程组.
③$\left\{\begin{array}{l}a-b=3 \\ \frac{1}{a}-3 b=4\end{array}\right.$中的第二个方程属于分式方程，它不属于二元一次方程．
④$\left\{\begin{array}{l}a+3 b=4 \\ 7 a-9 b=5\end{array}\right.$符合二元一次方程的定义．