In the Vicinity of a Bonfire the Temperature T In ${ } ^ { 0 } C$

Essay

In the vicinity of a bonfire the temperature T in ${ } ^ { 0 } C$
at a distance of x meters from the center of the fire was given by $T = \frac { 800,000 } { x ^ { 2 } + 400 }$
. At what range of distances from the fire's center was the temperature less than 400 ${ } ^ { 0 } C$
?