This can be determined by ellipse formula. Therefore, the height of the tunnel 4 feet from the edge is 13.08 feet.
Given:
A tunnel is constructed with a semi-elliptical arch. The width of the tunnel is 80 feet, and the maximum height at the center of the tunnel is 30 feet.
Find:
The height of the tunnel 4 feet from the edge is.
Let us consider,
The center of the concerned ellipse at the centre of its width of 80 feet. Let us consider
[tex]\begin{alligned}2a&=80\\\\a&=\dfrac{80}{2}&=\bold{40\:feet}\end{aligned}[/tex]
Since the maximum height at the centre of the tunnel is 30 feet, we can say that b = 30 feet.
Now, considering the centre of the tunnel as origin, we can write the equation of the ellipse as:
[tex]\bold{\dfrac{x^{2} }{a^{2} } +\dfrac{y^{2} }{b^{2} } =1}[/tex]
If we need to find the height of the tunnel 4 feet from the edge, we are looking for the height of the tunnel at 40 - 4 = 36 feet from the center of the tunnel.
Therefore, we need to find y from the equation of the ellipse, when x = 36 feet.Hence,
[tex]\begin{aligned}\dfrac{36^{2} }{40^{2} } +\dfrac{y^{2} }{30^{2} }&=1\\\dfrac{1296}{1600} +\frac{y^{2} }{900}&=1\\y^{2}&=(1-\frac{1296}{1600} )\times900\\y^{2}&=(1-0.81)\times900\\y^{2}&=0.19\times900\\y^{2}&=171\\y &=\sqrt{171\\\bold{y&=13.08}\end{aligned}[/tex]
Therefore, the height of the tunnel 4 feet from the edge is 13.08 feet.
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