Please help Ed!

Geometry Level 3

Ed wants to determine the height of a building.He holds a right triangular measuring device to his eyes and lines up the top of the triangle with the the top of the building and the bottom of the triangle with the base of the building.His eyes are 6ft above the ground and 30ft away from the building find the height of the building.

This picture doesn't necessarily represent the situation, round your answer to the nearest whole number.


The answer is 156.

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5 solutions

I have a different approach to this problem.

A figure drawn could help us, but don't worry, we can picture the scene in mind.

Let's draw a line from Ed's eye perpendicular to the wall. It will divide the bigger right angle triangle in two smaller right angle triangle. Say, the lower triangle has an angle θ \theta with the wall. So, the upper has 9 0 0 θ 90^0-\theta .

Clearly, tan θ = 30 6 = 5 \tan\theta=\frac{30}{6}=5 . So, tan ( 9 0 0 θ ) = cot θ = 1 5 \tan(90^0-\theta)=\cot\theta=\frac{1}{5} .

With these simple ratios, we can say, the portion of wall in the upper triangle is 5 5 times larger than the distance from Ed's eye to the wall. It's 150 , then.

Now now, the answer is 156 .

Using this approach, you could also just set up similar triangle and avoid all the trig completely. When I first did this I approached it with trig and got the answer, but your solution with the auxiliary line helped me see the similar triangles. Nice work!

Scott Immel - 7 years ago
Dingello Dan
Apr 19, 2014

Getting the hypotenuse of the lower triangle h = sqrt of (30^2 +6^2)

sin T = 30ft / h1 T = 78.69 degrees cos T = h1 / h2 h2 = 156

Marta Reece
Mar 25, 2017

Triangles A D B ADB and C D A CDA are similar, so 30 6 = x 6 30 \frac{30}{6}=\frac{x-6}{30} . Solve for x x to get x = 156. x=156.

Unstable Chickoy
Jun 6, 2014

let \(\h) be the height of the building

\(\cos\theta\ = \frac{height of his eye}{distance from his eye to the foot of the building} = \frac{6}{\sqrt{6^2 +30^2}} = \frac{\sqrt{6^2 +30^2}}{h}\)

6 6 2 + 3 0 2 = 6 2 + 3 0 2 h \frac{6}{\sqrt{6^2 +30^2}} = \frac{\sqrt{6^2 +30^2}}{h}

solving for \(\h)

\(h = \boxed{156}\)

Pierre Aïn
Apr 27, 2014

Can someone help ? Latex formatting doesn't work !!!

Let's consider 2 right triangles. One (called "Big") that Ed uses for measuring and one (called "Small") created between Ed, the ground and the building wall.

Let's consider "h" the Hypotenuse of the "Small" so that \­( { h }^{ 2 }={ 6 }^{ 2 }+{ 30 }^{ 2 }=936 \­)

Let's consider "H" the Hypotenuse of the Big

The two triangles have complementary angles.

Let's consider \­(\alpha \­) the angle of the corner of the small triangle where Ed is in positioned. \­(\alpha \­) is also the angle of the corner of the big triangle in contact with the bottom of the building.

Then \­( \cos ( \alpha )=\frac { 6 }{ h } =\frac { h }{ H } \­) Then \­( H=\frac { { h }^{ 2 } }{ 6 } =\frac { 936 }{ 6 } = \boxed{156} \­)

You can't include the = in the ( ... )\ because it's not part of the formatting you are trying to do. You have to do ( ... )\ = ( ... )\ while writing your equations. Also, read the formatting guide a little more carefully. Some of the format is wrong, for instance the {h}^{2} that you have wouldn't work because the format isn't correct. The correct format would be h^{2}. Hope you fix the problem.

Anthony Ng - 6 years, 11 months ago

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