Question

Three vertices of a parallelogram are shown in the figure below. Give the coordinates of the fourth vertex.

Answer 1

Given:

The three vertices of a parallelogram are (-3,8), (4,5), (2,-5).

To find:

The fourth vertex of the parallelogram.

Solution:

Let the vertices of the parallelogram are A(-3,8), B(4,5), C(2,-5) and D(a,b).

We know that, diagonals of a parallelogram bisect each other. It means midpoints of both diagonals are same.

Midpoint formula:

$Midpoint=\left(\dfrac{x_1+x_2}{2},\dfrac{y_1+y_2}{2}\right)$

Two diagonals of ABCD are AC and BD.

Midpoint of AC = Midpoint of BD

$\left(\dfrac{-3+2}{2},\dfrac{8-5}{2}\right)=\left(\dfrac{4+a}{2},\dfrac{5+b}{2}\right)$

$\left(\dfrac{-1}{2},\dfrac{3}{2}\right)=\left(\dfrac{4+a}{2},\dfrac{5+b}{2}\right)$

On comparing both sides, we get

$\dfrac{4+a}{2}=\dfrac{-1}{2}$

$4+a=-1$

$a=-1-4$

$a=-5$

And,

$\dfrac{5+b}{2}=\dfrac{3}{2}$

$5+b=3$

$b=3-5$

$b=-2$

Therefore, the coordinates of fourth vertex are (-5,-2).