✔ 最佳答案
1. 一正立方體加熱時, 各邊以 0.01 cm/h 的率增長. 當該正方體的邊長
為 20 cm時, 試求正立方體體積的變率?
Sol:
dx = 0.01
V = x^3
V’ = 3x² dx
= 3 * 20² * 0.01
= 12
Ans: 12cm^3/h
2. 燈塔L在一個小島上, 該小島與一筆直海岸線上最近點 A 相距 3 Km,
若燈塔的燈以每分鐘 2 周的率轉動,求射在海岸線上的光 P 在離 A 點
4 Km 處的速度?
Sol:
每分鐘轉 2 周, 一周係 2π
therefore, dθ/dt = 2 * 2π = 4π
Suppsoe B is x Km away from A
and the angle between PA and PB is θ
therefore,
tanθ = x/3
x = 3tanθ
dx/dt = 3 sec²θ * dθ/dt
題目已說明 x = 4
tanθ = 4/3
sec²θ = 1 + tan²θ
sec²θ = 1 + 16/9
sec²θ = 25/9
dx/dt = 3 sec²θ * dθ/dt
= 3 * 25/9 * 4π
= 100π/3
Ans: 100π/3 Km/min