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Geometry Level 3

I n a t r i a n g l e A B C , i t i s k n o w n t h a t A B = A C . S u p p o s e D i s t h e m i d p o i n t o f A C a n d B D = B C = 2 t h e n a r e a o f t r i a n g l e A B C w i l l b e In\quad a\quad triangle\quad ABC,\quad it\quad is\quad known\quad that\quad AB=AC\\ .Suppose\quad D\quad is\quad the\quad mid\quad point\quad of\quad AC\quad and\quad \\ BD=BC=2\quad then\quad area\quad of\quad triangle\quad ABC\quad will\quad be

Higher level of this problem is here


The answer is 2.64.

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5 solutions

Pratik Shastri
Nov 25, 2014

Here is a way to tackle the problem using vectors-



We are given that b = c \left|\vec{b}\right|=\left|\vec{c}\right| . Also, B C = B D = 2 |\vec{BC}|=|\vec{BD}|=2 .

B D = 2 c 2 b = 2 c 2 b 2 = 4 ( c 2 b ) ( c 2 b ) = 4 r 2 = r r c 2 4 b c cos A 2 b c cos A 2 + b 2 = 4 5 c 2 4 c 2 cos A = 4 c = b cos A = 5 4 4 c 2 \begin{aligned} |\vec{BD}|=2 \implies \left|\dfrac{\vec{c}}{2}-\vec{b}\right|&=2\\ \left|\dfrac{\vec{c}}{2}-\vec{b}\right|^2&=4\\ \left(\dfrac{\vec{c}}{2}-\vec{b}\right) \cdot \left(\dfrac{\vec{c}}{2}-\vec{b}\right)&=4 \ \ \ \ \ \ \ \ \ \ \because |\vec{r}|^2=\vec{r} \cdot \vec{r}\\ \dfrac{\left|\vec{c}\right|^2}{4}-\dfrac{\left|\vec{b}\right| |\vec{c}| \cos{A}}{2}-\dfrac{\left|\vec{b}\right| |\vec{c}| \cos{A}}{2}+\left|\vec{b}\right|^2&=4\\ \dfrac{5|\vec{c}|^2}{4}-|\vec{c}|^2 \cos{A}&=4 \ \ \ \ \ \ \ \ \ \ \because \ |\vec{c}|=\left|\vec{b}\right|\\ \cos{A}&=\dfrac{5}{4}-\dfrac{4}{|\vec{c}|^2}\\ \end{aligned}


But, cos A = 2 c 2 4 2 c 2 \cos{A}=\dfrac{2|\vec{c}|^2-4}{2|\vec{c}|^2} due to the law of cosines .

So, 1 2 c 2 = 5 4 4 c 2 c 2 = 8 cos A = 3 4 sin A = 7 4 1-\dfrac{2}{|\vec{c}|^2}=\dfrac{5}{4}-\dfrac{4}{|\vec{c}|^2} \implies |\vec{c}|^2=8 \implies \cos{A}=\dfrac{3}{4} \implies \sin{A}=\dfrac{\sqrt{7}}{4}


Lastly, Area of A B C = 1 2 b × c = 1 2 8 7 4 = 7 \text{Area of} \ \triangle ABC=\dfrac{1}{2}\left|\vec{b} \times \vec{c}\right|=\dfrac{1}{2} \cdot 8 \cdot \dfrac{\sqrt{7}}{4}=\boxed{\sqrt{7}}

nice solution Pratik I used coordinate geometry letting coordinates be B(00) , c(20) and hence A (1 2t) giving D (1.5 ) leading to a simple equation in t which is the area of the triangle

Des O Carroll - 6 years, 4 months ago

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Thank you :)

Pratik Shastri - 6 years, 2 months ago

T w o i s o s c e l e s Δ s A B C B D C s i n c e t h e y h a v e t h e s a m e c o m m o n e q u a l C . A B 2 = B C D C , A B 2 = 2 A B 2 . A B = 2 2 . u s i n g H e r o s o n Δ A B C , S = 2 2 + 2 2 + 2 2 = 2 2 + 1. A r e a Δ A B C = ( 2 2 + 1 ) 1 1 ( 2 2 1 ) = 8 1 = 2.6458 Two ~ isosceles ~\Delta s ~ABC~\cong~BDC~ since~they~have~the~same~ common~equal~\angle C.\\ \therefore~\dfrac{AB}2=\dfrac{BC}{DC},~~\implies~\dfrac{AB}2=\dfrac 2{\frac{AB}2}.\\ \implies~AB=2\sqrt2. \\ \therefore~using~Hero's~on~\Delta~ABC, ~~S=\dfrac{2\sqrt2+2\sqrt2 + 2} 2=2\sqrt2+1.\\ Area~\Delta~ABC=\sqrt{(2\sqrt2+1)*1*1*(2\sqrt2-1)}=\sqrt{8 - 1}=~~~~\Large \color{#D61F06}{2.6458} O R \large OR

I f C = X , all angles of the isosceles Δ s a r e g i v e n i n s k e t c h . L e t A B = A C = 2 m . A p p l y i n g S i n L a w t o Δ s A D B a n d C D B , S i n ( 3 X 180 ) S i n ( 180 2 X ) = A D B D = m 2 = A D C D = S i n ( 180 2 X ) S i n ( X ) S i n ( 3 X ) S i n ( 2 X ) = S i n ( 2 X ) S i n ( X ) If ~ \angle C=X, \text{all angles of the isosceles }\Delta s~are~given~in~ sketch.~~~Let~AB=AC=2m.\\ Applying ~ Sin ~ Law ~to~\Delta s ~ ADB ~ and ~ CDB,\\ \dfrac{Sin(3X-180)}{Sin(180-2X)}=\dfrac {AD}{BD} =\dfrac m 2 =\dfrac {AD}{CD}=\dfrac {Sin(180-2X)}{Sin(X)}\\ \implies ~ \dfrac { - Sin(3X)}{Sin(2X)}=\dfrac {Sin(2X)}{Sin(X)}\\ . . . . . . . . . . . . . . . . . . . . N o t e : S i n ( 2 X ) = 2 S i n ( X ) C o s ( X ) , S i n ( 3 X ) = 3 S i n ( X ) 4 S i n ( X ) 3 . S u b s t i t u t i n g a n d s o l v i n g t a k i n g c a r e o f n e g a t i v e s i g n w e g e t X = 69.29 5 o . ....................Note:-~~Sin(2X)=2*Sin(X)*Cos(X),~~Sin(3X)=3*Sin(X) - 4*Sin(X)^3.\\ Substituting ~ and ~ solving ~ taking ~ care ~ of ~ negative ~ sign ~we ~ get~~X= 69.295^o.\\ Since D is the midpoint of AC, a r e a Δ A B C = a r e a 2 Δ B D C = 2 2 S i n ( X ) 2 C o s ( X ) = 2.6458 \text{Since D is the midpoint of AC, } area~ \Delta~ABC=area~2*\Delta~BDC=2*2*Sin(X)*2*Cos(X)\\ =~~~~\Large \color{#D61F06}{2.6458}

Rudresh Tomar
Nov 13, 2014

L e t A B = A C = x D C = A D = x 2 N o w a c c . t o A p o l l o n i u s t h . : B C 2 + A B 2 = 2 ( B D 2 + D C 2 ) 4 + x 2 = 2 ( 4 + x 2 2 ) x = 2 2 a r ( Δ ) = a 4 b 2 a 2 4 [ w h e r e a = 2 , b = x ] a r ( Δ ) = 7 Let\quad AB=AC=x\quad \quad \quad \therefore \quad DC=AD=\frac { x }{ 2 } \\ Now\quad acc.\quad to\quad Apollonius\quad th.:\\ { \Longrightarrow \quad BC }^{ 2 }+AB^{ 2 }=2(BD^{ 2 }+{ DC }^{ 2 })\\ \Longrightarrow \quad 4+{ x }^{ 2 }=2(4+\frac { x^{ 2 } }{ 2 } )\\ \Longrightarrow \quad x\quad =\quad 2\sqrt { 2 } \\ \Longrightarrow \quad ar(\Delta )=\frac { a\sqrt { 4b^{ 2 }-a^{ 2 } } }{ 4 } \quad \quad \quad \quad [where\quad a=2,b=x]\\ \Longrightarrow \quad ar(\Delta )=\sqrt { 7 } \\ \\

how come BD and BC be equal , some logic is missing..... , plse reply...

Praveen Kumar - 6 years, 7 months ago

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it is given in the question

Rudresh Tomar - 6 years, 6 months ago

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can u plse make a diagram for that question and send me plse... , iam totally confused by that diagram....

Praveen Kumar - 6 years, 6 months ago
Md Rahman
Apr 7, 2015

apolanius,, BC^2+AB^2=2 (BD^2+DC^2) 4+(2DC)^2=2 (4+DC^2) 4+4DC^2=8+2DC^2 2DC^2=4 so, DC=2√2 and the rest is easy

Subh Mandal
Feb 7, 2015

ac=sec A triangles abc similar to cbd sec A/2=2/(sec A/2) sec^2A=8 secA= 2 2^1/2=ac therefore altitude is root 7 by hypotenuse thm. area is 1/2 2*root7 answer is root 7 =2.645

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