A regular pentagon is inscribed in a circle of radius 5 units . P is any point inside the pentagon. Perpendiculars are dropped from P to the sides, or the sides produced, of the pentagon.
The sum of the lengths of these perpendiculars can be expressed in the form p m n + o , where these are all positive integers such that o and p are coprime, and n is not divisible by the square of any prime.
Find the sum : m + n + o + p .
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The last step is incorrect, you forgot to multiply by r and divide by 4 :
5 × 5 × 4 5 + 1 = 4 2 5 5 + 2 5 , so the answer should be of the form s p q + r .
Please fix it, I liked this problem.
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Bro, 1st of all I'm truly sorry for such a silly error committed on my part. Can you please tell me what was the answer that you had entered previously so that I can make sure you get your rightful points back .
Thank you that you like my problem .
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My first answer was 55 and the second was 59 assuming that the answer is of the form s p q + r
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@Alan Enrique Ontiveros Salazar – Will you please explain how you got the value of 'r' i.e. the distance of vertex from geometric centre as 5 .
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@A Former Brilliant Member – You stated that the radius is 5. Is that what you asked me?
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@Alan Enrique Ontiveros Salazar – Sorry bro I got confused since I tried solving using In radius and I mistakenly thought that u did by the same .
Ignore me .
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@A Former Brilliant Member – Bro, I have reported the problem and soon you'll get your points .
Thanks. I have updated the answer.
Those who answered 15 were marked incorrect. Those who answered 55 or 59 were marked correct. The answer is now 59.
Thanks @Azhaghu Roopesh M for fixing the problem and solution :)
Yes! I used the extension of Viviani's Theorem
Each perpendicular to a side, and the side forms a triangle.
The perpendicular is the altitude of a triangle the side is the base.
Five such triangle cover full area of the pentagon.
However the same 5 sides with their apothem also cover the full area of the pentagon.
So the sum of the five apothem = sum of the perpendiculars.
Sum of 5 Apothem
=
5
∗
R
∗
c
o
s
3
6
=
5
∗
5
∗
4
5
+
1
=
4
2
5
5
+
2
5
=
p
m
n
+
o
.
m+n+o+p=25+5+25+4=59.
since P is any point, hence take p to be the centre of the circle and hence ans=5Rcos(pi/5)
Used the same method as yours. :D
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Let the pentagon be ABCDE, and the perpendiculars from P to sides AB, BC, CD, DE, and EA have lengths p 1 , p 2 , p 3 , p 4 , a n d p 5 , respectively.
Since P is inside the pentagon, we have that the area of the pentagon is the sum of the areas of the triangles ABP, BCP, CDP, DEP, and EAP. This sum is simply equal to
[ A B C D E ] = 2 1 ( A B ⋅ p 1 + B C ⋅ p 2 + C D ⋅ p 3 + D E ⋅ p 4 + E A ⋅ p 5 ) .
Now, since ABCDE is a regular pentagon, we have that all of its sides are equal to some positive s, and therefore
p 1 + p 2 + p 3 + p 4 + p 5 = 2 [ A B C D E ] / s .
Let O be the center of the pentagon. We know that ∠ A O B = 7 2 ° , so [ A O B ] = ( 1 / 2 ) r 2 sin 7 2 ° . We were given that ABCDE is a regular pentagon, so it follows that
p 1 + p 2 + p 3 + p 4 + p 5 = ( 5 r 2 / s ) sin 7 2 ° .
We now find s in terms of r. We know that ∠ O A B = 5 4 ° , s o [ A O B ] = ( 1 / 2 ) r s sin 5 4 ° . Therefore s = r ( sin 7 2 ° / sin 5 4 ° ) . This shows that
p 1 + p 2 + p 3 + p 4 + p 5 = 5 r sin 5 4 ∘ .
5 × 5 × 4 5 + 1
Therefore m = 2 5 , n = 5 , o = 2 5 , p = 4 ; Hence m + n + o + p = 5 9 .