方程①\(xy = 1\)②\(y = 3x - 1\)③\(x + \frac{1}{y} = 2\)④\(x + y + z = 1\)中，是二元一次方程的

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方程①\(xy = 1\)②\(y = 3x - 1\)③\(x + \frac{1}{y} = 2\)④\(x + y + z = 1\)中，是二元一次方程的有___

参考答案：②