题目

In America, just as in Europe, men usually open doors for women, and women always walk ahead of men into a room or a restaurant, unless the men have to be ahead of the ladies to choose the table, to open the door of a car or do some other things like the above. On the street, men almost always walk or cross the street on the closer side of the ladies to the traffic. But if a man walks with two ladies, he should walk between them. Then if the host or hostess or both of them come in a car to get their guest for dinner, the guest should sit at the front seat and leave the back seat though 下列各式：（1）4x+1=x-y，（2）1x+y=5，（3）x2-y2=2，（4）x=2y，（5）5x-3y，（6）x+y+z=1中属于二元一次方程的有______个．