题干

已知关于x,y的二元一次方程(a﹣1)x+(a+2)y+5﹣2a=0,当a每取一个值时,就有一个方程,而这些方程有一个公共解,试求出这个公共解.

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答案(点此获取答案解析)

解:将方程化为a的表达式:(x+y﹣2)a=x﹣2y﹣5,由于x,y的值与a的取值无关,即这个关于a的方程有无穷多个解,所以有x+y-2=0<

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阅读下列短文,从每题所给的四个选项(A、B、C和D)中选出最佳选项。

A

    When I was a little kid, a father was like the light in the fridge. Every house had one, but no one really knew what either of them did when the door was shut.

    My dad left the house every morning and always seemed glad to see every one again at night. He opened the jar of pickles when no one else at home could. He was the only one in the house who wasn't afraid to go into the basement by himself. 

    Whenever it rained, he got into the car and brought it around to the door. When anyone was sick, he went out to get the prescription(处方) filled. He set mousetraps. He cut back the roses so the thorns(刺) wouldn't hurt you when you came to the front door. He oiled my roller skates, and they went faster. When I got my bike, he ran alongside me for at least a thousand miles until I got the hang of it. He signed all my report and cards. He took a lot of pictures, but was never in them. He tightened up Mother's sagging(松垂的) clothesline every week or so.

    I was afraid of everyone else's father, but not my own. 

    Whenever I played house(玩过家家), the mother doll had a lot to do. I never knew what to do with the daddy doll, so I had him say, “I'm going off to work now,” and threw him under the bed.

    When I was nine years old, my father didn't get up one morning and go to work, he went to the hospital and died the next day. 

    There were a lot of people in the house who brought all kinds of good food and cakes. We had never had so much company before.   

    He never did anything; I didn't know his leaving would hurt so much.