### Still have math questions?

(a) Using the distance formula, find an expression for the square of the distance between the points $$P ( x , y )$$ and $$A ( 3,1 )$$ .
(b) Hence derive the equation of the locus of the point $$P ( x , y )$$ that moves so that it is always a distance of $$4$$ units from $$A$$ .
$$\left. \begin{array} { l l } { \text { a) } ( x - 3 ) ^ { 2 } + ( y - 1 ) ^ { 2 } } & { \text { (b) } ( x - 3 ) ^ { 2 } + ( y - 1 ) ^ { 2 } = 16 } \end{array} \right.$$