Consider the diagram below
Let AC be x ft and BC be y ft
"x^2+y^2=20^2\\\\\ny^2=20^2-x^2\\\\\n2y \\frac{dy}{dt}=-2x\\frac{dx}{dt}\\\\\nAlso \\\\\n\\frac{dx}{dt}=1 \\implies 2y \\frac{dy}{dt}=-2x*1 \\implies \\frac{dy}{dt}= -\\frac{x}{y}\\\\\ny^2=400-16 \\implies y =\\sqrt{384}\\\\\n\\frac{dy}{dt}= -\\frac{4}{\\sqrt{384}}=-0.2041"
Therefore the top of the ladder is moving downwards at 0.2041 ft/s
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