题目内容
一辆汽车沿平直公路行驶,开始以54km/h的速度行驶了全程的1/4,接着以速度v行驶其余的3/4的路程,已知全程的平均速度为72km/h,则v等于( )
A.22.5km/h | B.81km/h | C.80km/h | D.90km/h |
B
设全程的位移为4x,则汽车以速度v1=54km/h行驶了前x的位移,以速度v行驶了后3x的位移.
则汽车通过前
位移的时间t1=
;汽车通过后
位移的时间t2=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122817107444.png)
全程平均速度
=
代入解得:v=81km/h .
则汽车通过前
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122816827353.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122816842406.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122817029394.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122817107444.png)
全程平均速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122817123271.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122817279601.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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