Therefore, the distance between the helicopter and the soldier placed at (3,7) is
2
2
2
(
3)
(
7
7)
x
x
−
+
+
−
, i e , 
2
4
(
3)
x
x
−
+ Let
f (x) = (x – 3)2 + x4
or
f ′(x) = 2(x – 3) + 4x3 = 2(x – 1) (2x2 + 2x + 3)
Thus, f ′(x) = 0 gives x = 1 or 2x2 + 2x + 3 = 0 for which there are  no real roots