一辆运输车以每小时60公里的速度从A驶往B地,2小时后到达B地,并停留了30分钟来卸货.然后卡车以每小时80公里的速度返回A地.问：试建立卡车与A地的距离y（公里）与时间t（时间）之间的函数关系式,并析出定义域；2）当卡车离开A地3小时的时候,卡车离A地的距离是多少.第一个我知道是y=60t 第二个y=120 是什么意思啊 还有回去的函数我也不懂俄,求讲解亚~~~~~~~

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1） ①y=60t ( 0≤ t ≤ 2)
②y=120 (2 ≤ t ≤ 2.5)
③y=120-80(t-2.5) (2.5 ≤ t ≤ 4)
（2）t=3代入③得y=80
y=120的意思是卡车2小时后到达B地，并停留了30分钟，停留半小时A，B的距离就是每小时60公里所走的2小时的路程
③y=120-80(t-2.5) (2.5 ≤ t ≤ 4)
从B点返回时距A的距离

AB两地距离为：60*2=120公里
当0

y=60*2-（3-2-0.5）*80
60*2是A和B之间的距离，问3个小时候车距A多远 用B到A的距离减去卡车往回走 的距离
就得出Y了