Explanation
$${\textbf{Step - 1: Writing equations using distance formula}}$$
$${\text{Let the point which is equidistant from A(a,0,0), B(0,b,0), C(0,0,c) and O(0,0,0) be P(x, y, z)}}$$
$$\therefore {\text{ AP = BP = CP = OP}}$$
$$ \Rightarrow {\text{ }}\sqrt {{{\left( {{\text{x - a}}} \right)}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}} {\text{ = }}\sqrt {{{\text{x}}^{\text{2}}}{\text{ + }}{{\left( {{\text{y - b}}} \right)}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}} {\text{ = }}\sqrt {{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\left( {{\text{z - c}}} \right)}^{\text{2}}}} {\text{ = }}\sqrt {{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}} $$
$${\text{Squaring all sides,}}$$
$$ \Rightarrow {\text{ }}{\left( {{\text{x - a}}} \right)^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{\left( {{\text{y - b}}} \right)^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{\left( {{\text{z - c}}} \right)^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}$$
$$ \Rightarrow {\text{ }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}{\text{ - 2ax + }}{{\text{a}}^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}{\text{ - 2by + }}{{\text{b}}^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}{\text{ - 2zc + }}{{\text{c}}^{\text{2}}}{\text{ = }}{{\text{x}}^{\text{2}}}{\text{ + }}{{\text{y}}^{\text{2}}}{\text{ + }}{{\text{z}}^{\text{2}}}$$
$$ \Rightarrow {\text{ }}{{\text{a}}^{\text{2}}}{\text{ - 2ax = }}{{\text{b}}^{\text{2}}}{\text{ - 2by = }}{{\text{c}}^{\text{2}}}{\text{ - 2cz = 0}}$$
$${\textbf{Step - 2: Solving for x, y and z}}$$
$$\because {\text{ }}{{\text{a}}^{\text{2}}}{\text{ - 2ax = }}{{\text{b}}^{\text{2}}}{\text{ - 2by = }}{{\text{c}}^{\text{2}}}{\text{ - 2cz = 0}}$$
$$\therefore {\text{ }}{{\text{a}}^{\text{2}}}{\text{ - 2ax = 0}}$$
$$ \Rightarrow {\text{ 2ax = }}{{\text{a}}^{\text{2}}}$$
$$ \Rightarrow {\text{ x = }}\dfrac{{\text{a}}}{{\text{2}}}$$
$${\text{Similarly, }}{{\text{b}}^{\text{2}}}{\text{ - 2by = 0}}$$
$$ \Rightarrow {\text{ 2by = }}{{\text{b}}^{\text{2}}}$$
$$ \Rightarrow {\text{ y = }}\dfrac{{\text{b}}}{{\text{2}}}$$
$${\text{Similarly, }}{{\text{c}}^{\text{2}}}{\text{ - 2cz = 0}}$$
$$ \Rightarrow {\text{ 2cz = }}{{\text{c}}^{\text{2}}}$$
$$ \Rightarrow {\text{ z = }}\dfrac{{\text{c}}}{{\text{2}}}$$
$${\text{So the point is }}$$ $$\left( {\dfrac{{\text{a}}}{{\text{2}}}{\text{, }}\dfrac{{\text{b}}}{{\text{2}}}{\text{, }}\dfrac{{\text{c}}}{{\text{2}}}} \right)$$
$$\textbf{Hence option D is correct}$$
The coordinates of the vertices of the triangle are given by $$(1,2,-3) , (b,0,1), (-1,1,-4)$$
Accordingly the coordinates of the centroid of this triangle will be given by
($$ \dfrac{b}{3}, 1 , -2 $$)
Hence, $$ \dfrac{b}{3} $$ $$= 1$$ Or, $$b= 3$$
and $$a = -2$$
So $$a- b = -5$$
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