※ 引述《sandy871116 (珊珊)》之銘言：
: https://i.imgur.com/sKrQuQh.jpg
: 請問21題計算過程謝謝
k = 240, h = 320, v_2 = 720, v_1 = 960
B(0, k), v_B = -v_2j
A(h, 0), v_A = -v_1i
(a)
L^2 = (k - v_2t)^2 + (h - v_1t)^2
2L dL/dt = -2v_2(k - v_2t) - 2v_1(h - v_1t)
到這一步顯然看得出來一般情況下dL/dt是t的函數
但是題目給的數值剛好使得
v_2 = 3k
v_1 = 3h
所以
dL/dt = [-6k^2 (1 - 3t) - 6h^2 (1 - 3t)]/[2 * (1 - 3t) * √(k^2 + h^2)]
= -3√(k^2 + h^2)
= -1200 (km/hr)
(b)
k - v_2t = 0
=> t = k/v_2 = 1/3 (hr) = 20 (min)
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