Tangle with triangle

Geometry Level 5

Let Δ A B C \Delta ABC be a triangle with vertices A , B & C A,~B~\& ~C and D , E & F D,~E~\&~F be the midpoint of sides A B , B C & C A AB,~BC~\&~CA respectively.

If A E = 2.5 \color{#69047E}{AE~=~2.5} , C D = 13 \color{#EC7300}{CD~=~\sqrt{13}} and B F = 73 2 \color{#20A900}{BF~=~\dfrac{\sqrt{73}}{2}} , and P P be the image(reflection) of point A A about side B C BC .

Then find the area of the quadrilateral A B P C \color{#D61F06}{ABPC}

Note the given picture is not necessarily correct.

Try more of my geometry here .


The answer is 12.000.

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3 solutions

Nihar Mahajan
Apr 11, 2015

Area of triangle A B C ABC where u , v , w u,v,w are its medians is given by :

1 3 [ 2 ( u 2 v 2 + v 2 w 2 + u 2 w 2 ) ( u 4 + v 4 + w 4 ) ] \dfrac{1}{3}\sqrt{[2(u^2v^2+v^2w^2+u^2w^2)-(u^4+v^4+w^4)]}

Substituting the values of medians , we get

A ( A B C ) = 6 A(ABC)=6

Since , one of vertices of triangles is refected , it can be proved easily by congruence that :

A ( A B C ) = A ( B P C ) = 6 A(ABC)=A(BPC)=6

A ( A B C P ) = A ( A B C ) + A ( B P C ) = 12 \Rightarrow A(ABCP)=A(ABC)+A(BPC)=\huge\boxed{\color{#3D99F6}{12}}

@Aniket Verma @Purushottam Abhisheikh . Friends you can get this problem very easily . Thanks!

Nihar Mahajan - 6 years, 2 months ago

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Ya actually while thinking this question I forgot this formula and proceeded in a bit lengthy way that's why i posted it in level 5. BTW thanks for reminding me about this formula...

Aniket Verma - 6 years, 2 months ago

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@Vaibhav Prasad how did you do it?

Adarsh Kumar - 6 years, 2 months ago

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@Adarsh Kumar @Adarsh Kumar The area of a triangle = 4 3 \frac{4}{3} times the area of the triangle formed by it's medians.

Vaibhav Prasad - 6 years, 2 months ago

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@Vaibhav Prasad Ok!Thanx for replying!Sorry for the late reply,power was out since yesterday 8 30 PM.

Adarsh Kumar - 6 years, 2 months ago

@Adarsh Kumar I used the formula that median of a triangle Δ A B C \Delta ABC through its vertex A A is equal to b 2 2 + c 2 2 a 2 4 \sqrt{\dfrac{b^2}{2}~+~\dfrac{c^2}{2}~-~\dfrac{a^2}{4}}

And formed three equations to find the sides of the triangle...

Quite lengthy ...

Aniket Verma - 6 years, 2 months ago

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@Aniket Verma Oh!ok got it!

Adarsh Kumar - 6 years, 2 months ago

Hey! Do u remember posting a question and deleting it soon afterwards ? (It happened a few days ago)

Check out it's disputes .

A Former Brilliant Member - 6 years, 2 months ago

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Which question? Give me the link...

Nihar Mahajan - 6 years, 2 months ago

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Hey!Nihar,did you copy that combi question of yours (that 21,31,41..) from 102 combinatorial problems book?

Adarsh Kumar - 6 years, 2 months ago

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@Adarsh Kumar Yeah! But I didn't "copy" it , I "shared" it.

Nihar Mahajan - 6 years, 2 months ago

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@Nihar Mahajan ha ha

upvoted

Vaibhav Prasad - 6 years, 2 months ago

@Nihar Mahajan Yes,of course.

Adarsh Kumar - 6 years, 2 months ago

I did not get the link :/ @Azhaghu Roopesh M

Nihar Mahajan - 6 years, 2 months ago

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OK , no problem . Just participate in OPC 3 and I'll get back to u later.

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member You are not participating in OPC 3? :( :( :(

Nihar Mahajan - 6 years, 2 months ago

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@Nihar Mahajan IDK , still got some exams to go . The deadline's the 15th right ? I'll see .

If u are on G+, u can catch up with me . I'll be checking thaat out once every day .

Cya!

A Former Brilliant Member - 6 years, 2 months ago
Rohit Sachdeva
Apr 16, 2015

I used coordinate geometry

Let A(0,a) B(-b,0) & C(c,0)

Then E((c-b)/2,0) D(-b/2,a/2) & F(c/2,a/2)

Using diatance formula we get following equations :

b²+c²-2bc+4a²=25....(1)

4b²+c²+4bc+a²=73....(2)

b²+4c²+4bc+a²=52....(3)

From (2) & (3),

b²-c²=7 &

5b²+5c²+8bc+2a²=125

5(b²+c²-2bc)+18bc+2a²=125

5(25-4a²)+18bc+2a²=125

a²=bc

Plugging this in (1) we get (b+c)²=25 or b+c=5

Hence b-c=7/5

b=16/5, c=9/5 & a=12/5

Area of ABC is 1/2 x (b+c) x a= 6

Area of quad ABPC is 2ABC=12

Hrishik Mukherjee
Apr 13, 2015

There is another formula which can be applied to find out the area of a triangle. Let A be = (m1 + m2 + m3)/2, where m1, m2, m3 are the medians of the triangle. Then area of the triangle is given by the following expression:

4/3 x [(A)(A - m1)(A - m2)(A - m3)]^1/2

Sorry guys, still did not learn LaTex :(

I have the best way to learn LaTeX \LaTeX !

Just hover ur cursor over LaTeX \LaTeX codes of others and u'll be a master at it in no time !

A Former Brilliant Member - 6 years, 2 months ago

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@Azhaghu Roopesh M Yup thats a good idea.. By the way, you are there in the Brilliant Hangouts group right? We shall talk there :)

Hrishik Mukherjee - 6 years, 2 months ago

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Well , I had to leave it due to some reasons , add me back in !

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member Okay.. I will do it tomorrow

Hrishik Mukherjee - 6 years, 2 months ago

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