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[Maths] Section B Questions

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Section B Questions

How to find A'C?
Does the shadow possesses the same angle?
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AB=50m
tan30'=AB/A'B
A'B=86.6m


Angle ABC=130'
Given angle XAB=40'
YBA=40' (alt angle , XA//NB)
Cosine Laws
AC^2=AB^2+BC^2-2(AB)(BC)cos130'
???????????
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angleA'BC=90 ,because ABC is standing vertically on the ground, so angle ABC is 90. And angle A'BC is the shadow of ABC so angleA'BC is also 90.

Obviously, if you have found the length of A'B, you can use the pyth. thm. to find A'C

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引用:
原帖由 nearly_crazy 於 2010-4-5 09:29 PM 發表
angleA'BC=90 ,because ABC is standing vertically on the ground, so angle ABC is 90. And angle A'BC is the shadow of ABC so angleA'BC is also 90.

Obviously, if you have found the length of A'B, ...
I calucated just like you said when i attempt this question,but it was wrong.
The model answer is 108.

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i know what's wrong!!! The shadow is not necessarily 90, because... Umm... you may conduct an experiment to see if it's right. When we place a piece of paper under light, we can see the  shadow is not always vertically reflect on the ground.

Also in this question, the sun rays are not parallel to north, so the shadow of ABC is not 90.


Can you find the answer now? ;)
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