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anemone
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In triangle ABC, $\angle ACB=\angle ABC=80^\circ$ and $P$ is on the line segment $AB$ such that $BC=AP$. Find $\angle BPC$.
Amer said:Let $AB = 2x = AC$ then $AP = PB = x$, use the cosine law in the triangle $APC$ we found that
$PC^2 = x^2 + 4x^2 - 2(x)(2x) \cos 20 = x^2 ( 5 - 4\cos 20)\Rightarrow PC = x \sqrt{5 - 4\cos 20} $
again
$AC^2 = PC^2 + AP^2 - 2(PC)(AP) \cos \angle APC$
$3x^2 = x^2 ( 5 -4 \cos 20) + - 2x^2 \sqrt{5 - 4 \cos 20} \cos \angle APC $
$3 = 5 - 4 \cos 20 - 2 \sqrt{5 - 4 \cos 20} \cos \angle APC $
$\cos APC = \frac{1 - 2\cos 20}{\sqrt{5 - 4 \cos 20}}= - 0.789$
$\angle BPC = 180 - \arccos( - 0.789 ) $
I understand the question in a wrong way I will edit my post :)anemone said:Hi Amer! Thanks for participating but I'm sorry...your answer isn't correct...:(
Albert said:Geometric Solution:
johng said:I'm quite certain I know the answer, but I can't prove it. If the following is the way you went, I'd appreciate a hint.
When I carefully draw the figure, I see that the circum center O of triangle PBC has the property that triangle OBC is equilateral. From this the desired angle is obvious, but why is OBC equilateral?
from my diagram point E is on segment ACjohng said:Albert,
I believe you have made an error in your proof. Here's why I think this:
The 80-80-20 triangle has a very rich structure, and is the source of many good problems. I remember coming across some of them when I was a teenager, 60 years ago. I found this particular problem on the internet, here, with links to seven different solutions. They are all interesting, especially Solutions #1–#3, which all use different (ingenious and elegant) geometric constructions.anemone said:In triangle ABC, $\angle ACB=\angle ABC=80^\circ$ and $P$ is on the line segment $AB$ such that $BC=AP$. Find $\angle BPC$.
please see my post #10johng said:Albert,
My apologies. In your diagram, I didn't understand why |PE|=|BC| (I still can't see this). I then got all excited and drew the wrong diagram.
OpalG, I read the first three solutions in your link and I agree completely that they are elegant and ingenious. Thanks much for the link.
The given information is that Triangle ABC has two angles, ACB and ABC, that are both equal to 80 degrees.
The measure of angle BPC cannot be determined with the given information. Additional information about the triangle's side lengths or other angles is needed to find the measure of angle BPC.
The sum of the angles in any triangle is always 180 degrees. So, in Triangle ABC, the sum of angles ACB, ABC, and BCA would be 180 degrees.
No, Triangle ABC cannot be classified as a right triangle because none of its angles measure 90 degrees, which is a requirement for a triangle to be classified as a right triangle.
Yes, Triangle ABC can be classified as an isosceles triangle because it has two angles, ACB and ABC, that are equal to 80 degrees, making the two corresponding sides also equal in length.