Explanation:
(B) : Given,
The co-ordinates of mid points of a triangle
\(\mathrm{ABC}\) are \((1,5,-1),(0,4,-2)\) and \((2,3,4)\)

Let us assume the co-ordinate of \(\mathrm{A}, \mathrm{B}\) and \(\mathrm{C}\) are \(\left(\mathrm{x}_{1}, \mathrm{y}_{1}, \mathrm{z}_{1}\right),\left(\mathrm{x}_{2}, \mathrm{y}_{2}, \mathrm{z}_{2}\right)\) and \(\left(\mathrm{x}_{3}, \mathrm{y}_{3}, \mathrm{z}_{3}\right)\) respectively. Now,
We know that,
\(\therefore \quad\left[\frac{\mathrm{x}_{1}+\mathrm{x}_{2}}{2}, \frac{\mathrm{y}_{1}+\mathrm{y}_{2}}{2}, \frac{\mathrm{z}_{1}+\mathrm{z}_{2}}{2}\right]=(2,3,4)\)
\(\left.\frac{\mathrm{x}_{2}+\mathrm{x}_{3}}{2}, \frac{\mathrm{y}_{2}+\mathrm{y}_{3}}{2}, \frac{\mathrm{z}_{2}+\mathrm{z}_{3}}{2}\right]=(1,5,-1)\)
\({\left[\frac{\mathrm{x}_{1}+\mathrm{x}_{3}}{2}, \frac{\mathrm{y}_{1}+\mathrm{y}_{3}}{2}, \frac{\mathrm{z}_{1}+\mathrm{z}_{3}}{2}\right]=(0,4,-2)}\)
\(\text { Now, } \quad \frac{\mathrm{x}_{1}+\mathrm{x}_{2}}{2}+\frac{\mathrm{x}_{2}+\mathrm{x}_{3}}{2}+\frac{\mathrm{x}_{1}+\mathrm{x}_{3}}{2}=1+2+0\)
\(\therefore \quad \frac{2\left(\mathrm{x}_{1}+\mathrm{x}_{2}+\mathrm{x}_{3}\right)}{2}=3\)
\(\mathrm{x}_{1}+\mathrm{x}_{2}+\mathrm{x}_{3}=3\)
\(\mathrm{x}_{3}=3-\left(\mathrm{x}_{1}+\mathrm{x}_{2}\right)\)
\(=3-4\)
\({\left[\because \frac{\mathrm{x}_{1}+\mathrm{x}_{2}}{2}=2\right.}\)
\(\mathrm{x}_{3}=-1\)
\(\frac{x_{2}+x_{3}}{2}=1\)
\(\mathrm{x}_{2}+\mathrm{x}_{3}=2\)
\(x_{2}-1=2 \quad \Rightarrow \quad x_{2}=3\)
\(\mathrm{x}_{1}=0-\mathrm{x}_{3} \quad \Rightarrow \quad \mathrm{x}_{1}=1\)
Similarly, \(\frac{\mathrm{y}_{1}+\mathrm{y}_{2}}{2}=3, \frac{\mathrm{y}_{2}+\mathrm{y}_{3}}{2}=5, \frac{\mathrm{y}_{3}+\mathrm{y}_{1}}{2}=4\)
\(\therefore \quad 2\left(\mathrm{y}_{1}+\mathrm{y}_{2}+\mathrm{y}_{3}\right)=24\)
\(\mathrm{y}_{1}+\mathrm{y}_{2}+\mathrm{y}_{3}=12\)
\(\mathrm{y}_{3}=6, \mathrm{y}_{2}=4, \mathrm{y}_{1}=2\)
and, \(\quad \frac{\mathrm{z}_{1}+\mathrm{z}_{2}}{2}=4, \frac{\mathrm{z}_{2}+\mathrm{z}_{3}}{2}=-1, \frac{\mathrm{z}_{3}+\mathrm{z}_{1}}{2}=-2\)
\(\mathrm{z}_{1}+\mathrm{z}_{2}+\mathrm{z}_{3}=1\)
\(\therefore \quad \mathrm{z}_{1}=3, \quad \mathrm{z}_{2}=5, \quad \mathrm{z}_{3}=-7\)
\(\therefore \quad \mathrm{A}=(1,2,3)\)
\(\mathrm{B}=(3,4,5)\)
\(\mathrm{C}=(-1,6,-7)\)
Now,
Co-ordinate of centroid
\(\mathrm{G} =\left[\frac{\mathrm{x}_{1}+\mathrm{x}_{2}+\mathrm{x}_{3}}{3}, \frac{\mathrm{y}_{1}+\mathrm{y}_{2}+\mathrm{y}_{3}}{3}, \frac{\mathrm{z}_{1}+\mathrm{z}_{2}+\mathrm{z}_{3}}{3}\right]\)
\(=\left[\frac{1+3+(-1)}{3}, \frac{2+4+6}{3}, \frac{3+5-7}{3}\right]\)
\(=[1,4,1 / 3]\)