請教各位前輩高手以下這題：
A fish is reeled in along the water surface at a rate of 0.4m/s
from a point 4m above the surface.
At what rate is the angle θ between the line and the water changing
when there is a total of 10m line from the end of the rod to the water?
/
/∣＼
/ ∣ ＼
-----/ ∣ ＼ x （這裡標了一個 x ，但我想不出這要幹嘛？）
∣ ＼
4m∣ ＼
∣ θ ＼
----------------------------- 答案是 （2根號21）/525
謝謝各位
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