Trig Problem (1 Viewer)

[Sarah182]

I'm not really sure how to do this. It might be something so simple and I have missed it but I'm not sure.

David walks along a straight road. At one point he notices a tower on a bearing of 053* with an angle of elavation of 21*. After walking 230 metres the tower is on a bearing of 342*, with an angle of elavation of 26*. Find the height of the tower.

The answer is 84 metres. Help?

 

[naisAtoN]

Is that question from Maths in Focus?

I got exactly the same question today and my teacher had NO idea how to solve it. He spent over 50 minutes trying to solve it and got absolutely nowhere with it. It was such a waste of time - I have no idea how I would go about doing a question like that in an exam - I'd have to miss it because obviously I don't have an hour to spend on a question which is probably worth 3 marks.

Does anybody know how to go about doing it?

 

[michael1990]

You have to draw yourself a diagram. Working out the angles, by using the bearings.

You will then need to use.

Either c^2=a^2+b^2-2abCosC
Then you square root your answer to the above
The you have your answer.

c^2 is your side you're trying to find. a^2 and b^2 are the sides you already know.

~OR~
​

a = b
Sin A Sin B

a b being the sides opposite each angle underneath. Sin A and Sin B are the sides opposite those sides. (basic equation)

I didn't really read the question, but from the information given i think you have to use the Sin rule.

Also, i didn't draw a diagram.​

 

[naisAtoN]

Neither of those rules worked straight out, basically most of our class drew out diagrams and we tried everything we could think of; nothing worked.

 

[michael1990]

I just asked my Maths teacher he said its a very difficult question and he couldn't be bothered to sit down and do it as he has a class. If he gets time he will do it.

(I am using his computer in his office).

 

[A High Way Man]

i got 88.4

 

[fernando]

i got 88.4

you're a failure.

 

[tacogym27101990]

回复: Trig Problem

its not as hard as it looks
people are probably just gettin confused by the bearings?
the angle subtended at the foot of the pole is 53 +(360 -342) = 71

then just used the cos rule with one of the other sides of the triangle htan64 and the other htan69.
so u end up with

230² = htan64² + htan69² - 2*tan64*tan69*cos71

then just take h² out the front and whack it in your calc

would be easier to explain if i knew how to put diagrams on here

 

[lyounamu]

h = height of the tower

230/sin 71 = x/sin 72
x = 230/sin 71 . sin 72
= 231.347119...

However, tan 21 = h/x
Therefore, h = x . tan 21
= 231.347119... . tan 21
= 88.8058387...
= 88.81

This question is invalid. There are two approaches to solve this problem. However, both have different answer where they should all have equal answer.

 

[Devouree]

Yay! Some1 actually bothered to prove it! I did the question 2 times, and got the same answer... but i don't accept defeat very well so I tried the sin rule. Didn't work either. So I thought maybe the question was wrong... but obviously I'm not as confident in my abilities as you are

 

[tommykins]

Either answers are wrong or the question is weird.

Did it twice, differing answers.

 

[Aplus]

Question is wrong.

 

[lolokay]

the question is fine

just find the ratio of the distances from david to the pole at each point, sub into the cosine formula and then usual trig stuff

 

[Sarah182]

I'm guessing that the answers were wrong. It was a suss question, I mean its from Maths In Focus what do you expect.

 

[lolokay]

84 is the right answer

edit: I can draw up a diagram and stuff if you want




can lyanmou, or whoever else was getting around 88 show me how they got it?

 

[lyounamu]

84 is the right answer

edit: I can draw up a diagram and stuff if you want

Why were we given bearings then?

Post up your answer here.

 

[Sarah182]

84 is the right answer

edit: I can draw up a diagram and stuff if you want

That would be awesome but you dont need to go to that much trouble if you dont want to.

 

[tommykins]

Haha, working on one triangle I get 85, then the other triangle I get 51.56 - awesome.

 

[lolokay]

That would be awesome but you dont need to go to that much trouble if you dont want to.

already did




now can someone show me a valid diagram with a different answer? =/

Why were we given bearings then?

Post up your answer here.

in my working, to get the angle for the cosine formula (71)

 

[lyounamu]

already did

[URL="http://img223.imageshack.us/img223/7103/trigworking001re2.jpg"]http://img223.imageshack.us/img223/7103/trigworking001re2.jpg[/URL]


now can someone show me a valid diagram with a different answer? =/



in my working, to get the angle cosine formula (71)

Ok. I see what you did. I shouldn't have assumed it then (the angles of the triangle).