+782 Points $A,$ $B,$ and $C$ are given in the coordinate plane. There exists a point $Q$ and a constant $k$ such that for any point $P$,
PA^2 + PB^2 + PC^2 = 3PQ^2 + k.
If $A = (7,-11),$ $B = (10,13),$ and $C = (18,-22)$, then find the constant k.
+129771 Let P = (x,y)
PA^2 + PB^2 + PC^2 =
(x-7)^2 + (y + 11)^2 + (x-10)^2 + (y-13)^2 + (x-18)^2 + ( y + 22)^2 .....simplifying, we have
3 x^2 - 70 x + 3 y^2 + 40 y + 1247
3 [ x^2 - 70/3 x + y^2 + 40/3 y ] + 1247 complete the square on x,y
3[ x^2 - 70/3 x + 4900/36 + y^2 + 40/3y + 1600/36 ] + 1247 - 4900/12 -1600/12
3 [ (x - 70/6]^2 + (y + 40/6)^2 ] + 2116 / 3
Q = (70/6, -40/6 ) = (35/3 , - 20/3)
k = 2116 / 3
