Giải hệ phương trình (bằng phương pháp đặt ẩn phụ):
a) \(\left\{ \begin{array}{l}\frac{1} + \frac{1} = 2\\\frac{2} - \frac{3} = 1\end{array} \right.\)
b) \(\left\{ \begin{array}{l}\frac{3} - \frac{6} = - 1\\\frac{1} - \frac{3} = 0\end{array} \right.\)
c) \(\left\{ \begin{array}{l}\frac{1}{x} + \frac{1}{y} = \frac{1}{4}\\\frac{x} + \frac{1}{y} = 1\end{array} \right.\)
d) \(\left\{ \begin{array}{l}\frac{2} + \frac{3} = \frac{1}{2}\\\frac{2} - \frac{1} = \frac{1}\end{array} \right.\)
