※ 引述《Etienne (真是够了)》之铭言： : A man 6 feet tall walks at a rate of 5 feet per second : away from a light that is 15 feet above the ground. : When he is 10 feet from the base of the light, : (a)at what rate is the tip of his shadow moving? : (b)at what rate is the length of his shadow changing? : ~ ▎ ▎ ▏ ￣￣￣￣ 图有点难画...将就一点 我是先设白色那段长度为x 最底下整段(也就是蓝色+白色)长度为L (蓝色部份为影子) 黄色为灯的位置 红色为人的位置 (1) x = vt (v为速度 依题意为5ft/s ; t为此人从灯底下到现在位置经过的时间) (2) L = (5x)/3 (简单的用几何比一下就出来了) 答: (a)要求的是 dL/dt (b)要求的是 d(L-x)/dt 这题应该是难在它是应用题....吗? ※ 编辑: northerngaia 来自: 218.165.75.17 (11/14 08:57)