If the coordinates of the mid-points of the sides of a triangle be (3, −2), (−3, 1) and (4, −3), then find the coordinates of its vertices.
The co-ordinates of the midpoint $\left(x_{m}, y_{m}\right)$ between two points $\left(x_{1}, y_{1}\right)$ and $\left(x_{2}, y_{2}\right)$ is given by,
$\left(x_{m}, y_{m}\right)=\left(\left(\frac{x_{1}+x_{2}}{2}\right),\left(\frac{y_{1}+y_{2}}{2}\right)\right)$
Let the three vertices of the triangle be $A\left(x_{A}, y_{A}\right), B\left(x_{B}, y_{B}\right)$ and $C\left(x_{C}, y_{C}\right)$.
The three midpoints are given. Let these points be $M_{A \delta}(3,-2), M_{B C}(-3,1)$ and $M_{C C}(4,-3)$.
Let us now equate these points using the earlier mentioned formula,
$(3,-2)=\left(\left(\frac{x_{A}+x_{A}}{2}\right),\left(\frac{y_{A}+y_{B}}{2}\right)\right)$
Equating the individual components we get,
$x_{A}+x_{B}=6$
$y_{A}+y_{B}=-4$
Using the midpoint of another side we have,
$(-3,1)=\left(\left(\frac{x_{B}+x_{C}}{2}\right),\left(\frac{y_{B}+y_{C}}{2}\right)\right)$
Equating the individual components we get,
$x_{B}+x_{C}=-6$
$y_{B}+y_{C}=2$
Using the midpoint of the last side we have,
$(4,-3)=\left(\left(\frac{x_{A}+x_{C}}{2}\right),\left(\frac{y_{A}+y_{C}}{2}\right)\right)$
Equating the individual components we get,
$x_{A}+x_{C}=8$
$y_{A}+y_{C}=-6$
Adding up all the three equations which have variable ‘x’ alone we have,
$x_{4}+x_{B}+x_{3}+x_{f}+x_{4}+x_{f}=6-6+8$
$2\left(x_{A}+x_{B}+x_{C}\right)=8$
$x_{A}+x_{B}+x_{C}=4$
Substituting $x_{B}+x_{C}=-6$ in the above equation we have,
$x_{1}+x_{3}+x_{c}=4$
$x_{1}-6=4$
$x_{A}=10$
Therefore,
$x_{A}+x_{C}=8$
$x_{C}=8-10$
$x_{C}=-2$
And
$x_{A}+x_{B}=6$
$x_{s}=6-10$
$x_{B}=-4$
Adding up all the three equations which have variable ‘y’ alone we have,
$y_{A}+y_{B}+y_{B}+y_{C}+y_{A}+y_{C}=-4+2-6$
$2\left(y_{A}+y_{B}+y_{C}\right)=-8$
$y_{A}+y_{B}+y_{C}=-4$
Substituting $y_{B}+y_{C}=2$ in the above equation we have,
$y_{A}+y_{B}+y_{C}=-4$
$y_{A}+2=-4$
$y_{A}=-6$
Therefore,
$y_{A}+y_{C}=-6$
$y_{c}=-6+6$
$y_{C}=0$
And
$y_{A}+y_{B}=-4$
$y_{B}=-4+6$
$y_{B}=2$
Therefore the co-ordinates of the three vertices of the triangle are $A(10,-6)$ $B(-4,2)$ $C(-2,0)$ .