Given that the inverse trigonometric function assumes principal values only. Let x, y be any two real numbers in [-1, 1] such that -1x - -1y = , - 2 . Then, the minimum value of x2 + y2 + 2xy is
- 0
- -1
- 12
- -12
Answer
(1) 0
Solution
-1y = 2 - -1y ⇒ -1x + -1y = 2 +
Take cos: xy - 1 - x2 1 - y2 = ( 2 + ) = -
xy + = 1 - x2 1 - y2
Square: x2y2 + 2xy + 2 = 1 - x2 - y2 + x2y2
x2 + y2 + 2xy = 2 0
Minimum = 0 (at = 2, e.g. x = y = 0)
Answer: (1)