007A Sample Midterm 2, Problem 5

A kite 30 (meters) above the ground moves horizontally at a speed of 6 (m/s). At what rate is the length of the string increasing when 50 (meters) of the string has been let out?

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Solution:
550px
Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle 30^{2}+h^{2}=s^{2}}
$s=50$
Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle {\begin{array}{rcl}\displaystyle {h}&=&\displaystyle {\sqrt {s^{2}-30^{2}}}\\&&\\&=&\displaystyle {\sqrt {50^{2}-30^{2}}}\\&&\\&=&\displaystyle {40}\end{array}}}
$2hh'=2ss'$
$2(40)6=2(50)s'$
Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle {\begin{array}{rcl}\displaystyle {s'}&=&\displaystyle {\frac {2(40)(6)}{2(50)}}\\&&\\&=&\displaystyle {{\frac {24}{5}}{\text{ m/s}}}\end{array}}}

Detailed Solution

Foundations:
The Pythagorean Theorem
For a right triangle with side lengths  $a,b,c$  where  $c$  is the length of the
hypotenuse, we have  $a^{2}+b^{2}=c^{2}.$

Step 1:
550px
From the diagram, we have  $30^{2}+h^{2}=s^{2}$  by the Pythagorean Theorem.
Taking derivatives, we get
Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle 2hh'=2ss'.}

Step 2:
If   Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle s=50,}   then
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h=\sqrt{50^2-30^2}=40.}
So, we have
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2(40)6=2(50)s'.}
Solving for   Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle s',}   we get   Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle s'=\frac{24}{5} \text{ m/s.}}

Final Answer:
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle s'=\frac{24}{5} \text{ m/s}}

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