AP微积分题目.

题目

AP微积分题目.
A street light is at the top of a 12.0 ft. tall pole. A man 5.0 ft tall walks away from the pole with a speed of 4.5 feet/sec along a straight path. How fast is the tip of his shadow moving when he is 42 feet from the pole?
只给答案也可以.为什么我反复算都说45/14但是不对.

答案

设从灯到人的距离为x,人到灯影子最前端距离为y用相似三角形得5/12=y/x+y所以5/7=y/x也就是x=7y/5已知x=4.5t所以4.5t=7y/5等号两边同时关于t求导,得4.5=(7/5)*dy/dt所以dy/dt=45/14但是,dy/dt并不是影子最前端移动的...