Is the Given Information Sufficient?

Geometry Level 4

Let O O be a point inside A B C \triangle ABC such that

O C O E = 10 3 and O A O D = 9 4 . \dfrac{OC}{OE}=\frac{10}{3} ~~~\text{and} ~~~\dfrac{OA}{OD}=\frac{9}{4}.

If

B E E A + B D D C = m n , \dfrac{BE}{EA}+\dfrac{BD}{DC}=\dfrac{m}{n},

where gcd ( m , n ) = 1 \gcd(m,n)=1 , find the value of m + n 2 m+n-2 .


The answer is 11.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

5 solutions

Deepanshu Gupta
Nov 4, 2014

For my Solution One must needed to know some physics !!

So Put the Point masses at the various points .

Put mass 3 kg at C and now using the concept that mass moment about the centre of mass of an system are equal so at point E we must have to put mass of 10 kg

So Mass at the COM of CE is 13 kg which is present at the point O

Similarly at point 'A' mass would be 4 kg and at the point 'D' it is 9 kg

So by equating mass moment about point 'E'

B E × 6 = E A × 4 B E E A = 4 6 . . . . ( 1 ) BE\quad \times \quad 6\quad =\quad EA\quad \times \quad 4\\ \\ \cfrac { BE }{ EA } \quad =\quad \cfrac { 4 }{ 6 } \quad \quad \quad \quad .\quad .\quad .\quad .\quad \quad (1) .

And by equating mass moment about point 'D'

B D × 6 = D C × 3 B D C D = 3 6 . . . . ( 2 ) BD\quad \times \quad 6\quad =\quad DC\quad \times \quad 3\\ \\ \cfrac { BD }{ CD } \quad =\quad \cfrac { 3 }{ 6 } \quad \quad \quad \quad .\quad .\quad .\quad .\quad \quad (2) .

By adding 1 and 2 we get

B D C D + B E E A = 7 6 \\ \cfrac { BD }{ CD } \quad +\quad \cfrac { BE }{ EA } \quad =\quad \cfrac { 7 }{ 6 } .

m = 7 m = 7 .

n = 6 n= 6 .

WoW !! Physics in maths I can't even imagine !!

Karan Shekhawat - 6 years, 7 months ago

Log in to reply

Why not? And actually we can mathematically define and prove all "physics" terms and facts we use, so it has nothing to do with physics at all. I used the same method, BTW.

All we need to do is to put masses a , b and c to the points A , B and C correspondingly, so that pairs have their centers of mass at points D and E while all three have the center at O .

Masses can be easily found from two conditions:

3a + 3b = 10c and 4b + 4c = 9a

then a = 4 , b = 6 and c = 3 is one of the solutions. After that we can easily find all other ratios remembering all masses from pairs are coming to to their centers.

Anatoliy Razin - 6 years, 7 months ago

Log in to reply

My concept of mass moment is not clear when can I get it ....s plz suggest some links

Kundan Patil - 6 years, 4 months ago

Menelaus theorem is also a way.

mietantei conan - 6 years, 7 months ago

Nice.. I used vector algebra to solve the problem. I thought there was no way to avoid it :)

Pratik Shastri - 6 years, 6 months ago

Interesting! I used similarity though.

Satvik Golechha - 6 years, 7 months ago

Log in to reply

extra lines and other garbage all over the place... yet still very nice :D

Nathan Ramesh - 6 years, 7 months ago

I made the question with the help of Cordinate Geometry .

Shubhendra Singh - 6 years, 7 months ago

i jst wanna know what does (COM) stand for ? .. great solution btw

amr elshabouny - 6 years, 7 months ago

Log in to reply

COM means "Center of mass" .

Deepanshu Gupta - 6 years, 7 months ago

Let the areas of A C O , A E O , E B O , B D O \triangle ACO, \triangle AEO, \triangle EBO, \triangle BDO and C D O \triangle CDO be a , b , c , d a,b,c,d and e e respectively. Then we have:

a b = O C O E = 10 3 b = 3 10 a . . . 1 \dfrac {a}{b}= \dfrac {OC}{OE}=\dfrac {10}{3}\quad \Rightarrow b = \frac {3} {10} a\quad ...1

d + e c = O C O E = 10 3 d + e 10 3 c = 0 . . . 2 \dfrac {d+e}{c}= \dfrac {OC}{OE}=\dfrac {10}{3}\quad \Rightarrow d+e - \frac {10} {3} c=0\quad ...2

a e = O A O D = 9 4 e = 4 9 a . . . 3 \dfrac {a}{e}= \dfrac {OA}{OD}=\dfrac {9}{4}\quad \Rightarrow e = \frac {4} {9} a\quad ...3

b + c d = O A O D = 9 4 b + c 9 4 d = 0 . . . 4 \dfrac {b+c}{d}= \dfrac {OA}{OD}=\dfrac {9}{4}\quad \Rightarrow b+c - \frac {9} {4} d=0\quad ...4

Substituting eq. 3 in eq. 2 and multiplying throughout by 9 4 \frac {9}{4} :

9 4 d + 9 4 4 9 a 9 4 10 3 c = 0 9 4 d + a 15 2 c = 0 . . . 5 \Rightarrow \frac {9}{4}d+\frac {9}{4}\frac {4}{9}a - \frac {9}{4}\frac {10} {3} c=0\quad \Rightarrow \frac {9}{4}d+a - \frac {15}{2}c=0 \quad ...5

Substituting eq. 1 in eq. 4:

3 10 a + c 9 4 d = 0 . . . 6 \Rightarrow \frac {3} {10} a+c - \frac {9} {4} d=0\quad ...6

Adding eq. 5 to eq. 6 13 10 a 13 2 ) c = 0 c = 1 5 a \Rightarrow \frac {13} {10} a - \frac {13}{2})c = 0 \quad \Rightarrow c = \frac {1}{5}a

Substituting c = 1 5 a c = \frac {1}{5}a in eq. 5:

9 4 d + a 15 2 1 5 a = 0 d = 2 9 a \Rightarrow \frac {9}{4}d+a - \frac {15}{2}\frac {1}{5}a=0\quad \Rightarrow d = \frac {2}{9}a

Now we have: B E E A + B D D C = c b + d e = 1 5 × 10 3 + 2 9 × 9 4 = 2 3 + 1 2 = 7 6 = m n \dfrac {BE}{EA} + \dfrac {BD}{DC} = \dfrac {c}{b} + \dfrac {d}{e} = \frac{1}{5} \times \frac{10}{3}+\frac{2}{9} \times \frac{9}{4} = \frac{2}{3} + \frac{1}{2} = \frac{7}{6} = \dfrac{m}{n}

Therefore, the required answer m + n 2 = 7 + 6 2 = 11 m+n-2=7+6-2=\boxed{11}

is there a proof that a/b = OC/OE ??

amr elshabouny - 6 years, 7 months ago

Log in to reply

Amr, please note that A C O \triangle ACO and A E O \triangle AEO has the same height or perpendicular distance between A A and C B CB . Let this height be h h , then we have:

The area of A C O = a = O C × h \triangle ACO= a = OC \times h

The area of A E O = b = O E × h \triangle AEO= b = OE \times h

Therefore, a b = O C × h O E × h = O C O E \dfrac {a}{b} = \dfrac {OC\times h}{OE\times h} = \boxed{\dfrac {OC}{OE}}

Chew-Seong Cheong - 6 years, 7 months ago
Kartik Sharma
Nov 10, 2014

Join BO and extend it to F on AC.

Now, I don't know how to prove it and I want proof of it too(maybe it needs mass point geometry or it can be done using Ceva's theorem too I guess.)

O D A D + O E C E + O F B F = 1 \frac{OD}{AD} + \frac{OE}{CE} + \frac{OF}{BF} = 1

3 13 + 4 13 + O F B F = 1 \frac{3}{13} + \frac{4}{13} + \frac{OF}{BF} = 1

O F B F = 6 13 \frac{OF}{BF} = \frac{6}{13}

O B O F = 7 6 \frac{OB}{OF} = \frac{7}{6}

Also, I saw the following somewhere(Wikipedia though) and want to know its proof

B O O F = B E E A + B D D C \frac{BO}{OF} = \frac{BE}{EA} + \frac{BD}{DC}

Hence, B E E A + B D D C = 7 6 \frac{BE}{EA} + \frac{BD}{DC} = \frac{7}{6}

PLEASE, I want both the proofs(I think Ceva's theorem will be enough but I am just not getting it.

Kartik Sharma - 6 years, 7 months ago

Log in to reply

Cevas theorem apply to the triangle BCE and vertices A,O,D,and once more Cevas theroem aplied to the triangle DBA and vertices A,O,D gives solution after minut or two calculating...

Nikola Djuric - 6 years, 6 months ago

1st formula hint: Ratio of the sides = Ratio of the area

2nd formula I'm still working with the proof. Might come back later.

Samuraiwarm Tsunayoshi - 6 years, 7 months ago

Log in to reply

Can you pls share both of them here? -

https://brilliant.org/discussions/thread/need-derviation-of-some-formulas-help/?auto login key=vmP7GpRs3iSjLclsIqkJdUq19ijGOweqRnsVmvFnAa#comment-a3f558715e92

Kartik Sharma - 6 years, 7 months ago

Nikola Djuric
Nov 30, 2014

Cevas theorem apply to the triangle BCE and vertices A,O,D,and once more Cevas theroem aplied to the triangle DBA and vertices A,O,D gives solution after minut or two calculating...

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...