Answer to Question #250810 in Calculus for Sam

Question #250810
10. A picture 40 cm high is placed on a wall with its base 30 cm above the level of the eye of an observer. If the observer is approaching the wall at the rate of 40 cm/sec, how fast is the measure of the angle subtended at the observer's eye by the picture changing when the observer is 1 m from the wall?
1
Expert's answer
2021-10-17T17:43:06-0400



"Angle \\; \u03b8 = \u03b8_2-\u03b8_1 \\\\\n\u03b8 = cot^{-1}(\\frac{x}{70}) -cot^{-1}(\\frac{x}{30}) \\\\\n\\frac{d\u03b8 }{dt} = \\frac{\\frac{-1}{70} \\frac{dx}{dt}}{1 + \\frac{x^2}{70^2}} - \\frac{\\frac{-1}{30} \\frac{dx}{dt}}{1 + \\frac{x^2}{30^2}} \\\\\n\\frac{d\u03b8 }{dt} = \\frac{-dx\/dt}{70}(\\frac{4900}{4900+x^2} + \\frac{dx-dt}{30}(\\frac{900}{900+x^2}) \\\\\n\\frac{dx}{dt} = -40 \\\\\nx = 100 \\;cm \\\\\n\\frac{d\u03b8 }{dt} = \\frac{-40}{70}\\frac{4900}{4900+10000} + \\frac{-40}{30}(\\frac{900}{900+10000}) \\\\\n\\frac{d\u03b8 }{dt} = 4(\\frac{700}{14900}) -4(\\frac{300}{10900}) \\\\\n\\frac{d\u03b8 }{dt} = \\frac{28}{149} - \\frac{12}{109} = 0.07782 \\;rad\/sec"

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