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Area of triangle and collinearity of three points - class-XI

Description: area of triangle and collinearity of three points
Number of Questions: 18
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Tags: matrix algebra mathematics and statistics matrices and determinants determinants maths inverse of a matrix and linear equations determinants and matrices
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If $\begin{vmatrix} x _1 & y _1 & 1 \ x _2 & y _2 & 1 \ x _3 & y _3 & 1\end{vmatrix}=\begin{vmatrix} a _1 & b _1 & 1\ a _2 & b _2 & 1 \ a _3 & b _3 & 1\end{vmatrix}$, then the two triangles with vertices $(x _1, y _1), (x _2, y _2), (x _3, y _3)$ and $(a _1,b _1)$, $(a _2, b _2)$, $(a _3, b _3)$ must be congruent.

  1. True

  2. False


Correct Option: B
Explanation:

The two determinants denote twice the area of $\Delta^s$ whose vertices are $(x _1,y _1), (x _2, y _2)$, $(x _3, y _3)$ and $(a _1, b _1)$, $(a _2, b _2), (a _3, b _3)$. This the equality of two determinants implies that their areas are equal. But equality of the areas of two triangles does not imply that they are congruent.

If the area of the triangle with vertices $(2, 5), (7, k)$ and $(3, 1)$ is $10$, then find the value of $k$.

  1. $-5$ or $35$

  2. $5$ or $-35$

  3. $15$ or $-5$

  4. $-5$ or $-25$


Correct Option: B
Explanation:
If $(x _1,y _1), (x _2, y _2)$ ans $(x _3, y _3)$ are the vertices of a triangle, then its area is given by $\pm \dfrac {1}{2}[x _1(y _2-y _3)+x _2(y _3-y _1)+x _3(y _1-y _2)]$ 
Given vertices are $(2,5), (7,k), (3,1)$ and area is $10$.
Therefore, $\pm 10 = \dfrac {1}{2}[2(k-1)+7(1-5)+3(5-k)]$
$\Rightarrow \pm 20=2k-2-28+15-3k$
$\Rightarrow \pm 20=-k-15$
$\Rightarrow k = 5$ or $-35$

If $\displaystyle \left | \begin{matrix}x _{1} &y _{1}  &1 \ x _{2} &y _{2}  &1 \ x _{3} &y _{3}  &1 \end{matrix} \right |=\left | \begin{matrix}1 &1  &1 \ b _{1} &b _{2}  &b _{3} \ a _{1} &a _{2}  &a _{3}\end{matrix} \right |$ then the two triangles whose vertices are $\displaystyle \left ( x _{1},y _{1} \right ), \left ( x _{2},y _{2} \right ), ( \left ( x _{3},y _{3} \right ) $ and $\displaystyle\left ( a _{1},b _{1} \right ), \left ( a _{2},b _{2} \right ), \left ( a _{13},b _{3} \right ),$ are

  1. congruent

  2. similar

  3. equal in area

  4. none of these


Correct Option: C
Explanation:

If $\left( x _{ 1 },y _{ 1 } \right) ,\left( x _{ 2 },y _{ 2 } \right) ,(\left( x _{ 3 },y _{ 3 } \right) $ are the vertices of triangle , then its area is 

$A _{1}=\dfrac { 1 }{ 2 } \left| \begin{matrix} x _{ 1 } & y _{ 1 } & 1 \ x _{ 2 } & y _{ 2 } & 1 \ x _{ 3 } & y _{ 3 } & 1 \end{matrix} \right| $

If $\left( a _{ 1 },b _{ 1 } \right) ,\left( a _{ 2 },b _{ 2 } \right) ,\left( a _{ 3 },b _{ 3 } \right) $ are the vertices of triangle , then its area is

$A _{2}=\dfrac { 1 }{ 2 } \left| \begin{matrix} a _{ 1 } & b _{ 1 } & 1 \ a _{ 2 } & b _{ 2 } & 1 \ a _{ 3 } & b _{ 3 } & 1 \end{matrix} \right| $

$A _{2}=\dfrac { 1 }{ 2 } \left| \begin{matrix} a _{ 1 } & a _{ 2 } & a _{ 3 } \ b _{ 1 } & b _{ 2 } & b _{ 3 } \ 1 & 1 & 1 \end{matrix} \right|    (\because |A|=|A^{T}|)$


$A _{2}=-\dfrac{1}{2}\left| \begin{matrix} 1 & 1 & 1 \ b _{ 1 } & b _{ 2 } & b _{ 3 } \ a _{ 1 } & a _{ 2 } & a _{ 3 } \end{matrix} \right| $

Since, area is positive,

$A _{2}=\dfrac{1}{2}\left| \begin{matrix} 1 & 1 & 1 \ b _{ 1 } & b _{ 2 } & b _{ 3 } \ a _{ 1 } & a _{ 2 } & a _{ 3 } \end{matrix} \right| $

Given, $\left| \begin{matrix} x _{ 1 } & y _{ 1 } & 1 \ x _{ 2 } & y _{ 2 } & 1 \ x _{ 3 } & y _{ 3 } & 1 \end{matrix} \right| =\left| \begin{matrix} 1 & 1 & 1 \ b _{ 1 } & b _{ 2 } & b _{ 3 } \ a _{ 1 } & a _{ 2 } & a _{ 3 } \end{matrix} \right| $

$\Rightarrow A _{1}=A _{2}$

Let O(0, 0), P(3,4), Q(6, 0) be the vertices of the triangle OPQ. The point R inside the triangle OPQ is such that the triangles OPR,PQR, OQR are of equal area. The coordinates of R are 

  1. $\displaystyle \left ( \frac{4}{3}, 3 \right )$

  2. $\displaystyle \left ( 3, \frac{2}{3} \right )$

  3. $\displaystyle \left ( 3, \frac{4}{3} \right )$

  4. $\displaystyle \left ( \frac{4}{3}, \frac{2}{3} \right )$


Correct Option: C
Explanation:

Let coordinate of $R = (a,b)$

Given area of triangle OPR, PQR, and OQR are same. 

$\cfrac{1}{2}\begin{vmatrix} 0&0&1\3&4&1\a&b&1\end{vmatrix}=\cfrac{1}{2}\begin{vmatrix} 3&4&1\6&0&1\a&b&1\end{vmatrix}=\cfrac{1}{2}\begin{vmatrix} 0&0&1\6&0&1\a&b&1\end{vmatrix}$

$\Rightarrow 3b-4a=24-4a-3b=6b$.

Solving this equation be get $a=3, b =\cfrac{4}{3}$

The co-ordinates of the vertices A, B, C of a triangle are $ \displaystyle \left ( 6,3 \right ),\left ( -3,5 \right ),\left ( 4,-2 \right ) $ respectively and P is any point $ \displaystyle \left ( x,y \right ), $ then the ratio of areas of triangles PBC and ABC is

  1. $ \displaystyle \begin{vmatrix}x-y-2\end{vmatrix}:7 $

  2. $ \displaystyle \begin{vmatrix}x+y+2\end{vmatrix}:7 $

  3. $ \displaystyle \begin{vmatrix}x+y-2\end{vmatrix}:7 $

  4. None of these


Correct Option: C
Explanation:
Let  $ P=(x,y)$

We have area of $\displaystyle \triangle PBC=\left| \frac { 1 }{ 2 } \begin{vmatrix} x\quad  & y\quad  & 1 \\ -3 & 5 & 1 \\ 4 & -2 & 1 \end{vmatrix} \right| $

$\displaystyle =\frac { 1 }{ 2 } \left| \left[ x\left( 5+2 \right) -3\left( -2-y \right) +4\left( y-5 \right)  \right]  \right| $

$\displaystyle =\frac { 1 }{ 2 } \left| 7x+7y-14 \right| =\frac { 7 }{ 2 } \left| x+y-2 \right| $

Area of $\displaystyle \triangle ABC=\left| \frac { 1 }{ 2 } \begin{vmatrix} 6\quad  & 3\quad  & 1 \\ -3 & 5 & 1 \\ 4 & -2 & 1 \end{vmatrix} \right| $

$\displaystyle =\frac { 1 }{ 2 } \left| \left[ 6\left( 5+2 \right) -3\left( -2-3 \right) +4\left( 3-5 \right)  \right]  \right| $

$\displaystyle \\ =\dfrac { 1 }{ 2 } \left| 42+15-8 \right| =\dfrac { 49 }{ 2 } $

$\displaystyle \therefore \frac { area\triangle PBC }{ area\triangle ABC } =\dfrac { \left| x+y-2 \right|  }{ 7 } $

if $ \displaystyle a,b,c $ as well as $ \displaystyle d,e,f $ are in G.P. with same common ratio then set of points $ \displaystyle \left ( a,d \right ),\left ( b,e \right ),\left ( c,f \right ) $ are

  1. collinear

  2. concurrent

  3. lies on a circle

  4. lie on an ellipse


Correct Option: A
Explanation:

Are of triangle formed by the given points is,
$\Delta  = \cfrac{1}{2}\left|\begin{vmatrix}a&d&1\b&e&1\c&f&1\end{vmatrix}\right|$
Let common ratio is $r$
$\Rightarrow \Delta = \cfrac{1}{2}\left|\begin{vmatrix}a&d&1\ar&dr&1\ar^2&dr^2&1\end{vmatrix}\right|$
taking $a$ and $d$ common from first and second column respectively,
$\Delta =  \cfrac{ad}{2}\left|\begin{vmatrix}1&1&1\r&r&1\r^2&r^2&1\end{vmatrix}\right| = 0$, Since first and second column are same.
Hence given points are collinear.

The vertices of the triangle $ABC$ are $(2, 1, 1), (3, 1, 2), (-4, 0, 1)$. The area of triangle is

  1. $\displaystyle \frac{3\sqrt{38}}{2}$

  2. $\sqrt{38}$

  3. $\displaystyle \frac{\sqrt{38}}{2}$

  4. $4$


Correct Option: C
Explanation:

The vertices of the triangle $ABC$ are $(2,1,1),(3,1,2),(-4,0,1)$
$\overrightarrow { AB } =i+k$ and $\overrightarrow { AC } =-6i-j$
now, $\displaystyle \triangle =\frac { \left| \overrightarrow { AB } \times \overrightarrow { AC }  \right|  }{ 2 } =\frac { \left| \left( i+k \right) \times \left( -6i-j \right)  \right|  }{ 2 } =\frac { \left| i-6j-k \right|  }{ 2 } $
Therefore, $\triangle =\dfrac { \sqrt { 38 }  }{ 2 } $

Ans: C

Let $\displaystyle A\left ( x _{1},y _{1} \right ),B\left ( x _{2},y _{2} \right ), C\left ( x _{3},y _{3} \right )$ be three points. Area of triangle with vertices $A, B,C$ is given by
$\displaystyle \frac{1}{2}\left | \Delta  \right |$ where,  

$\displaystyle \Delta =\begin{vmatrix}x _{1} &y _{1}  &1 \ x _{2} & y _{2}  & 1\ x _{3} &y _{3}  &1 \end{vmatrix}$.

If $\displaystyle a=BC,b=CA,c=AB$ and $\displaystyle 2s=a+b+c$, then $\displaystyle \Delta ^{2}$ equals

  1. $\displaystyle abc $

  2. $\displaystyle s(s-a)(s-b)(s-c)$

  3. $\cfrac {abc}{4} $

  4. $\displaystyle 4s(s-a)(s-b)(s-c)$


Correct Option: D
Explanation:
Given area$=\cfrac { 1 }{ 2 } \left| \triangle  \right| $

Heron's formula

Area$=\sqrt { S\left( S-a \right) \left( S-b \right) \left( S-c \right)  } $

$\cfrac { 1 }{ 2 } \left| \triangle  \right| =\sqrt { S\left( S-a \right) \left( S-b \right) \left( S-c \right)  } $

Squaring on both sides

$=\cfrac { { \left| \triangle  \right|  }^{ 2 } }{ 4 } =S\left( S-a \right) \left( S-b \right) \left( S-c \right) $

${ \left| \triangle  \right|  }^{ 2 }=4S\left( S-a \right) \left( S-b \right) \left( S-c \right) $

Option D

Let $\displaystyle A\left ( x _{1},y _{1} \right ),B\left ( x _{2},y _{2} \right ), C\left ( x _{3},y _{3} \right )$ be three points. Area of triangle with vertices $A, B,C$ is given by $\displaystyle \frac{1}{2}\left | \Delta  \right |$ where,  $\displaystyle \Delta =\begin{vmatrix}x _{1} &y _{1}  &1 \\
x _{2} & y _{2}  & 1\\
x _{3} &y _{3}  &1
\end{vmatrix}$.If $\displaystyle \triangle ABC$ is an equilateral triangle and $\displaystyle a = BC$ is a rational number, then $\displaystyle \triangle$ must be
  1. an integer

  2. a rational number

  3. an irrational number

  4. an imaginary number


Correct Option: C
Explanation:

If $a$ is rational then $a^{ 2 }$ is also rational 
Now as $\Delta =\dfrac { \sqrt { 3 }  }{ 4 } a^{ 2 }$
Then $\Delta $ is irrational

What is the area of the triangle formed by the points $(a,c+a), (a,c)$ and $(-a,c-a)$?

  1. $\displaystyle- a^{2}$

  2. $\displaystyle \frac{1}{a^{2}}$

  3. $\displaystyle a^{2}+a$

  4. zero


Correct Option: A
Explanation:

$\left( a,c+a \right)  \left( a,c \right)  \left( -a,c-a \right) $

$\triangle \begin{vmatrix} 1 & 1 & 1 \ a & a & -a \ c+a & \quad c & \quad c-a \end{vmatrix}$
$ac-{ a }^{ 2 }+ac-(ac-{ a }^{ 2 }+ac+{ a }^{ 2 })+ac-ac-{ a }^{ 2 }$
$-2{ a }^{ 2 }-2ac+2ac$
$-2{ a }^{ 2 }$
Area $=\cfrac { 1 }{ 2 } \left[ \triangle  \right] =\cfrac { 1 }{ 2 } \left( -{ a }^{ 2 } \right) $
$=-{ a }^{ 2 }$

What is the area of the triangle formed by the points $(a,c+a), \displaystyle \left ( a^{2},c^{2} \right )$ and $(-a, c-a)$?

  1. $1$

  2. $\displaystyle a^{2}$

  3. $\displaystyle \sqrt{a^{2}+c^{2}}$

  4. None of these


Correct Option: D
Explanation:

$(a,c+a)\quad ({ a }^{ 2 },{ c }^{ 2 })\quad (-a,\quad c-a)$

$\triangle =\begin{vmatrix} 1 & 1 & 1 \ a & { a }^{ 2 } & -a \ c+a & { \quad c }^{ 2 } & \quad c-a \end{vmatrix}$
${ a }^{ 2 }c-{ a }^{ 3 }+a{ c }^{ 2 }-ac+{ a }^{ 2 }-ac-{ a }^{ 2 }+a{ c }^{ 2 }+{ a }^{ 2 }c-{ a }^{ 3 }$
$2a{ c }^{ 2 }-2{ a }^{ 3 }-2ac$
Area $=\cfrac { 1 }{ 2 } \triangle $
$=a{ c }^{ 2 }-{ a }^{ 3 }-ac$
OPTION-D

What is the area of the triangle formed by the points $(a,b+c), (b,c+a)$ and $(c,a+b)$?

  1. $1$

  2. $-1$

  3. $0$

  4. $\displaystyle \frac{1}{2}\left ( abc \right )^{2}$


Correct Option: C
Explanation:
$\left( a,b+c \right) ;\left( b,c+a \right) ;\left( c,a+b \right) $
$\triangle =\begin{vmatrix} 1 & 1 & 1 \\ a & b & c \\ b+c & c+a & a+b \end{vmatrix}$
$ab+{ b }^{ 2 }-{ c }^{ 2 }-ac-{ a }^{ 2 }-ab+bc+{ c }^{ 2 }+ac+{ c }^{ 2 }-{ a }^{ 2 }-bc$
$=0$
Area of triangle$=\cfrac { 1 }{ 2 } \left| \triangle  \right| =0$
Option C

The area of a triangle whose vertices are (-2,-2), (-1,-3) and (p,0) is 3 sq.units what is the value of p?

  1. -2

  2. 2

  3. 3

  4. -3


Correct Option: B
Explanation:

Let the vertices of the triangle A(-2,-2),B(-1,-3) and C(P,0) then

$Area  of   \triangle ABC=\frac{1}{2}\left[x _1(y _2-y _3)+x _2(y _3-y _1)+x _3(y _1-y _2)\right]$
Area =3 sq. unit
Here$x _1=-2,y _1=-2$
        $x _2=-1,y _2=-3$
         $x _3=p,y _3=0$
$\Rightarrow 3=\frac{1}{2}\left[(-2(-3-0)+-1(0-(-2))+p(-2-(-3)\right]$
$\Rightarrow 3=\frac{1}{2}[6-2+p]$
$\Rightarrow 6=4+p$
$\Rightarrow -p=4-6$
$\Rightarrow p=2$

The area of a triangle, whose vertices are $(3, 2), (5, 2)$ and the point of intersection of the lines $x = a$ and $y = 5$, is $3$ square units. What is the value of $a$?

  1. $2$

  2. $3$

  3. $4$

  4. $5$


Correct Option: B
Explanation:
$A(3,2) B(5,2)$ intersection of $x=a,y=5$   $(a,5)$
$\cfrac { 1 }{ 2 } \left| \triangle  \right| =3$
$\triangle =6$
$=\begin{vmatrix} 1 & 1 & 1 \\ 3 & 5 & a \\ 2 & 2 & 5 \end{vmatrix}$
$=25-2a-15+2a+6-10$
$=6$
For any value of a area $=3sq.units$

If $P=(x _{1}, y _{1}), Q=(x _{2}, y _{2})$ and $R=(x _{3}, y _{3})$ are three points of a triangle in $\mathbb{R}^{2}$. Then, area of a $\triangle PQR$ in terms of determinant of matrix $M=\begin{bmatrix} 1& 1 & 1 \ x _{1} & x _{2} & x _{3} \ y _{1} & y _{2} & y _{3}\end{bmatrix}$ is

  1. $-|det(M)|$

  2. $|det(M)|$

  3. $\dfrac{1}{2}|det(M)|$

  4. $2|det(M)|$


Correct Option: C
Explanation:
$P=\left( { x } _{ 1 },{ y } _{ 1 } \right) ,Q=\left( { x } _{ 2 },{ y } _{ 2 } \right) ,R\left( { x } _{ 3 },{ y } _{ 3 } \right) $
$\left| M \right| =\triangle =\begin{vmatrix} 1 & 1 & 1 \\ { x } _{ 1 } & { x } _{ 2 } & { x } _{ 3 } \\ { y } _{ 1 } & { y } _{ 2 } & { y } _{ 3 } \end{vmatrix}$
Area of triangle$=\cfrac { 1 }{ 2 } det(M)$
Answer: Option C

If $\triangle _1,\triangle _2$ be the areas of two triangles with vertices $(b,c), (c,a), (a,b)$, and $ (ac-b^2, ab-c^2),(ba-c^2, bc-a^2), (cb-a^2, ca-b^2)$, then $\ \dfrac{\triangle _1}{\triangle _2}=(a+b+c)^2$

  1. True

  2. False


Correct Option: A
Explanation:

$T _1 = (b, c)\ (c, a)\ (a, b) = (x _1, y _1)\ (x _2, y _2)\ (x _3, y _3)$

$T _2 = [(ac - b^2), (ab  - c^2)], [(ba  - c^2), (bc - a^2)], [(cb - a^2), (ca - b^2)]$
$T _1 = \dfrac{1}{2} [c(c - a) + a(a - b) + b(b - c)]$
$= \dfrac{1}{2} [c^2 - ac + a^2 - ab + b^2 - bc]$
$T _2 = \dfrac{1}{2} [(ab - c^2) [ba - c^2 - cb + a^2] + (bc - a^2) [cb - a^2 - ac + b^2] + (ca - b^2) [ac - b^2 - ba + c^2)]$
$= \dfrac{1}{2} [[(ab - c^2)(a - c) (a + b + c)] + (bc - a^2) [(b - a)(a + b + c)] + (ca - b^2) [(c - b) (a + b + c)]]$
$= \dfrac{(a + b + c)}{2} [a^2b - abc - ac^2 + c^3 + b^2 c - abc - a^2b + c^2a - abc - b^2 c + b^3]$
$= \dfrac{(a + b + c)}{2} (a^3 + b^3 + c^3 - 3abc)$
$= \left(\dfrac{a + b + c}{2}\right) [a + b + c] [ a^2 + b^2 + c^2 - ab - bc - ca]$
$\therefore \ \dfrac{\Delta _1}{\Delta _2} = \dfrac{(a + b + c)^2 [a^2 + b^2 + c^2 - bc - ba - ca]}{(a^2 + b^2 + c^2 - bc - ba - ca)}$
$= (a + b + c)^2$

If ${ \left( { x } _{ 1 }-{ { x } _{ 2 } } \right)  }^{ 2 }+{ \left( { y } _{ 1 }-{ y } _{ 2 } \right)  }^{ 2 }={ a }^{ 2 }$, ${ \left( x _{ 2 }-{ x } _{ 3 } \right)  }^{ 2 }+{ \left( { y } _{ 2 }-{ y } _{ 3 } \right)  }^{ 2 }={ b }^{ 2 }$, ${ \left( { x } _{ 3 }-{ x } _{ 1 } \right)  }^{ 2 }+{ \left( { y } _{ 3 }-{ y } _{ 1 } \right)  }^{ 2 }={ c }^{ 2 }$ and $k\begin{vmatrix} { x } _{ 1 } & { y } _{ 1 } & 1 \ { x } _{ 2 } & { y } _{ 2 } & 1 \ { x } _{ 3 } & { y } _{ 3 } & 1 \end{vmatrix}=(a+b+c)(b+c-a)(c+a-b)\times (a+b-c)$, then the value of $k$ is

  1. $1$

  2. $2$

  3. $4$

  4. none of these


Correct Option: C
Explanation:
Given : $k\begin{vmatrix} { x } _{ 1 } & { y } _{ 1 } & 1 \\ { x } _{ 2 } & { y } _{ 2 } & 1 \\ { x } _{ 3 } & { y } _{ 3 } & 1 \end{vmatrix}^{2}=(a+b+c)(b+c-a)(c+a-b)\times (a+b-c)$ ...... $(i)$

Area of any triangle $PQR$ with vertices $(x _1, y _1) , (x _2, y _2) $ and $(x _3, y _3)$ is given by
$\Delta = \dfrac{1}{2} \begin{vmatrix}x _1 & y _1 & 1\\ x _2 & y _2 & 1\\ x _3 & y _3 & 1\end{vmatrix}$ ......... $(ii)$

${ \left( { x } _{ 1 }-{ { x } _{ 2 } } \right)  }^{ 2 }+{ \left( { y } _{ 1 }-{ y } _{ 2 } \right)  }^{ 2 }={ a }^{ 2 }$, ${ \left( x _{ 2 }-{ x } _{ 3 } \right)  }^{ 2 }+{ \left( { y } _{ 2 }-{ y } _{ 3 } \right)  }^{ 2 }={ b }^{ 2 }$, ${ \left( { x } _{ 3 }-{ x } _{ 1 } \right)  }^{ 2 }+{ \left( { y } _{ 3 }-{ y } _{ 1 } \right)  }^{ 2 }={ c }^{ 2 }$

$\implies a,b,c$ are the length of sides of a $\triangle PQR$

Also, area of $\triangle PQR$ with sides $a, b, c$ is

$\Delta = \sqrt{s (s - a) (s - b) (s - c)}$ ......... $\left[\text{Where } s=\dfrac{a+b+c}{2}\right]$

$= \displaystyle \sqrt{\frac{1}{16}(2s)(2s - 2a)(2s - 2b)(2s - 2c)}$

$\Delta = \displaystyle \sqrt{\left [ \frac{(a + b + c)(b + c - a)(c + a - b)(a + b - c)}{16}\right ]}$ ........... $(iii)$

From $(ii)$ and $(iii)$, we get

$\displaystyle \frac{1}{2} \begin{vmatrix} x _1&  y _1& 1\\ x _2 & y _2 & 1\\ x _3 & y _3 & 1\end{vmatrix} = \sqrt{\left [\frac{(a+b+c)(b+c- a)(c + a - b)(a + b - c)}{16} \right ]}$

Squaring both sides, we have

$\Rightarrow 4 \begin{vmatrix}x _1 & y _1 & 1\\ x _2 & y _2 & 1\\ x _3 & y _3 & 1\end{vmatrix}^2 = (a+b+c)(b+c-a)(c+a- b)(a+b-c)$

Comparing above equation with $(i)$, we get $k=4$

$(x _1 - x _2)^2 + (y _1 - y _2)^2 = a^2$;
$(x _2 - x _3)^2 + (y _2 - y _3)^2 = b^2$;
$(x _3 - x _1)^2 + (y _3 - y _1)^2 = c^2$;
then find $4 \begin{vmatrix}x _1 & y _1 & 1\ x _2 & y _2 & 1\ x _3 & y _3 & 1\end{vmatrix}^2 = $

  1. $(a+b+c) (b+c - a) (c + a - b) (a + b - c)$

  2. $-(a+b+c) (b+c - a) (c + a - b) (a + b - c)$

  3. $-(a+b+c) (b+c - a) (c + a - b) (a + b - c)/2$

  4. $(a+b+c) (b+c - a) (c + a - b) (a + b - c)/2$


Correct Option: A
Explanation:

Area of the triangle PQR with vertices $(x _1, y _1) , (x _2, y _2) $ and $(x _3, y _3)$ is

$\therefore

\Delta = \dfrac{1}{2} \begin{vmatrix}x _1 & y _1 & 1\ x _2 &

y _2 & 1\ x _3 & y _3 & 1\end{vmatrix}$          ....(1)

Now the area of $\Delta$ $PQR$ with sides $a, b, c$ is

$\Delta = \sqrt{s (s - a) (s - b) (s - c)}$

$= \displaystyle \sqrt{\frac{1}{16}(2s)(2s - 2a)(2s - 2b)(2s - 2c)}$

$\Delta = \displaystyle \sqrt{\left [ \frac{(a + b + c)(b + c - a)(c + a - b)(a + b - c)}{16}\right ]}$          ....(2)

From (1) and (2), we get

$\displaystyle

\frac{1}{2} \begin{vmatrix} x _1&  y _1& 1\ x _2 & y _2 &

1\ x _3 & y _3 & 1\end{vmatrix} = \sqrt{\left [

\frac{(a+b+c)(b+c- a)(c + a - b)(a + b - c)}{16} \right ]}$

Squaring both sides, we have

$\Rightarrow

4 \begin{vmatrix}x _1 & y _1 & 1\ x _2 & y _2 & 1\ x _3

& y _3 & 1\end{vmatrix}^2 = (a+b+c)(b+c-a)(c+a- b)(a+b-c)$

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