Answer to Question #248141 in Calculus for jhonh

Question #248141

Consider the given right triangle with legs of length x cm. and y cm. and angle θ radians. If x is decreasing at the rate of 3 cm/min. and y is increasing at the rate of 4 cm/min., at what rate is angle θ changing when x=5 cm. and y=2 cm. ?


1
Expert's answer
2021-10-12T02:05:29-0400

Solution:

"dy\/dt=4\\ cm\/min\n\\\\ dx\/dt=-3\\ cm\/min\n\\\\x=5\\ cm\n\\\\y=2\\ cm"

"\\sec\\theta=\\dfrac {\\sqrt{x^2+y^2}}{x}\n\\\\\\Rightarrow \\sec^2\\theta=\\dfrac {{x^2+y^2}}{{x^2}} \\ \\ ...(i)"

Also,

"\\tan \\theta=\\dfrac{AC}{AB}\n\\\\\\Rightarrow\\tan \\theta=\\dfrac yx"

On differentiating both sides w.r.t "t",

"\\sec^2\\theta \\dfrac{d\\theta}{dt}=\\dfrac{x(dy\/dt)-y(dx\/dt)}{x^2}\n\\\\\\Rightarrow\\dfrac {x^2+y^2}{{x^2}} \\dfrac{d\\theta}{dt}=\\dfrac{x(dy\/dt)-y(dx\/dt)}{x^2}\n\\\\ \\Rightarrow (\\dfrac{5^2+2^2}{5^2}) \\dfrac{d\\theta}{dt}=\\dfrac{5(4)-2(-3)}{5^2}"

"\\\\ \\Rightarrow (\\dfrac{29}{25}) \\dfrac{d\\theta}{dt}=\\dfrac{26}{25}\n\\\\ \\Rightarrow \\dfrac{d\\theta}{dt}=\\dfrac{26}{29} \\ radians\/min"


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