Let the maximum and minimum values of ( 8x - x2 - 12 - 4 )2 + (x - 7)2, x R be M and m, respectively. Then M2 - m2 is equal to .
Answer
1600
Solution
Put y = 8x - x2 - 12, so (x - 4)2 + y2 = 4 with y 0: the upper semicircle with centre C(4, 0) and radius 2.
The expression (x - 7)2 + (y - 4)2 = PA2 is the square of the distance of a point P of this semicircle from A(7, 4).
[figure]
CA = 32 + 42 = 5; since CA meets the semicircle at Q, the nearest point is Q, so m = (CA - 2)2 = 32 = 9.
The point of the full circle farthest from A lies below the x-axis, so on the semicircle the farthest point is the end B(2, 0): M = AB2 = 52 + 42 = 41.
M2 - m2 = 1681 - 81 = 1600
Answer: 1600