Geometry Marathon

Solution of Problem 12:

Suppose, the inradius of \triangle{ABC} is r and the circumradius of \triangle{ABC} is R.

If the circumcenter of \triangle{ABC} is located on AB, then ABC is right-angled triangle, AB is the hypotenuse of \triangle{ABC} and the circumcenter of \triangle {ABC} is on the midpoint of AB

.Given that, AB=29 , AC = 20.

According to Pythagoras theorem,
                          AC^2+BC^2=AB^2

                    => 20^2 + BC^2 = 29^2

                    => BC^2 = 841-400

                    => BC^2 = 441

                    => BC = 21

Now,      AB=29 , BC=21,AC=20,

\therefore The area of \triangle{ABC}=\cfrac{1}{2}*BC*AC=\cfrac{1}{2}*21*20=210

Again, s =\cfrac{AB+BC+AC}{2}= \cfrac{29+21+20}{2}=35

The length of inradius, r = \cfrac{area\space{of}\space{\triangle{ABC}}}{s} =\cfrac {210}{35}=6

The length of circumradius, R=\cfrac{29}{2}=14.5

The distance of incenter and circumcenter = \sqrt{R^2-2rR} = \sqrt{(14.5)^2-2*6*14.5} = \sqrt{210.25-174}=\sqrt{36.25}=6.020797.......

The distance of incenter and circumcenter = 6.02     \fbox{Ans}

In the figure, AB=CD=2 , AC=6 , EO=3 and BC and big circle is tangent to 2 small equal circle. What is the length of EH? (Problem 13)

মনে করি EH, AC কে M বিন্দুতে ছেদ করে।
AM = MC = \frac{6}{2} = 3
AB = MH = CD = 2
AE = 2+3 = 5

\Delta AME সমকোণী ত্রিভুজ
AE^2 = AM^2 + ME^2
ME = \sqrt{5^2 - 3^2}= 4
EH = MH+ ME = 4+2 = 6
\fbox {EH = 6}(ans)
(no problem to submit :slight_smile: )

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I think here is a typing mistake.

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yeah typing mistake. now it’s ok :slight_smile: .

একটি ত্রিভুজ \triangle ABC-এর পরিকেন্দ্র হলো O এবং লম্ববিন্দু হলো PBPOC একটি বৃত্তস্থ চতুর্ভুজ। \angle BAC-এর মান কত? (Problem 14)

No replies…

Are we dead? :thinking:


Let M be the center of the circle passing through B, P, O, and C.

Because P is the orthocenter of ABC, the two segments AB and BP are hypotenuses for two right triangles in the figure above. Using those triangles, we find that \angle BAC and \angle BPD, shown above in dark green, are both complementary to \angle ABP and therefore congruent to each other.
The Inscribed Angle Theorem does the heavy lifting:
(1) \triangle BPC \cong \triangle BOC because both are inscribed in a circle about M.
(2)\angle BAC has half their measure, as it’s the inscribed angle corresponding to central angle \angle BOC.
Now \angle BPD is supplementary to \angle BPD and has half its measure. Therefore \angle BPD has measure 60^{\circ} , as must \angle BAC .

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Great!!!

Can you pls post the next problem…

(Problem 15)

Source: Numberphile

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Here,
AB=BC=CD=AD=BE=EF=FC=EG=GM=MF=MN=NK=KF=KL=LC=LJ=JD

So,
\triangle AED = \triangle KNG

So, \angle DEA + \angle DGA = \angle KGN + \angle DGA

Here, \angle KGN + \angle DGA + \angle DGK = 90 \degree
So, \angle DGA+ \angle DEA + \angle DGK = 90 \degree

In \triangle DGK,
DK = KG so, \angle KDG = \angle KGD = 45 \degree

So, \angle KGN + \angle DGA + 45 \degree = 90 \degree
So, \angle KGN + \angle DGA = 45 \degree
So, \angle DEA + \angle DGA = 45 \degree

In \triangle ABD,
AD = AB so, \angle ADB= \angle ABD = 45 \degree

So, \angle DEA + \angle DGA + \angle ABD= 45 \degree + 45 \degree = 90 \degree

So, \angle DBA + \angle DEA + \angle DGA = 90 \degree

প্রমাণ করো যে, এমন কোনো সুষম ষড়ভুজ নেই যাদের প্রতিটা শীৰ্ষই একটা বর্গাকার গ্রিডের বিন্দু। (Problem 16)

Solution of problem 16:

মনে করি, সুষম ষড়ভুজের প্রতিটি শীর্ষবিন্দুই বর্গাকার গ্রিডের কোনো একটি বিন্দু।
এবারে, প্রতিটি বাহুর উপর সমান দৈর্ঘ্যের লম্ব আঁকি।

এখানে, প্রতিটি ধাপে ষড়ভুজ সমান অনুপাতে ছোট হচ্ছে এবং একটা সময় তা শুন্য হয়ে যাবে। কিন্তু, কখনোই ষড়ভুজের আকার এর বাহুর উপর আঁকা বর্গের আকারের চেয়ে ছোট হয় না।

অতএব, এমন কোনো ষড়ভুজ নেই, যার প্রতিটি শীর্ষবিন্দুই বর্গাকার গ্রিডের বিন্দু। [প্রমাণিত]

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Can you pls post the next problem?

How about opening a number theory marathon

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Please, solve the unsolved problem!

Can’t solve it…

We should jump to the next problem…

@ROHAN1230 এটাও গনিতযজ্ঞের প্রব্লেম। তাই উত্তর দেওয়া নিষেধ।

@adnansgphs No I have collected this problem from a book…

But still sorry I am changing…

Let n \geq 2 be a positive integer, and let A, B, and C be points in n-dimensional space. Given that \angle ABC=\angle ACB, prove or disprove that AB=AC. (Problem 17)