✔ 最佳答案
通常咁做包冇錯:
1. -1 <= sin x <= 1
==> 5 >= -5 sin x >= -5
==> 2+5 >= 2-5 sin x >= 2-5
分兩個 cases,
7 >= 2-5 sin x >= 0 或 0 >= 2-5 sin x >= -3
==> 49 >= (2-5 sin x)² >= 0 或 0 <= (2-5 sin x)² <= 9
==> 0 <= (2-5 sin x)² <= 49
所以最大值係 49,最細值係 0。
2. 0 <= cos²x <= 1
==> 0 >= -cos²x >= -1
==> 6 >= 6-cos²x >= 6-1
==> 5 <= 6-cos²x <= 6
所以最大值係 6,最細值係 5。
3. -1 <= -sin x <= 1
==> 3-1 <= 3-sin x <= 3+1
==> 1/2 >= 1/(3-sin x) >= 1/4
所以最大值係 1/2,最細值係 1/4。
(注意:跨過正負值時要分開兩個 cases )
2015-04-05 18:20:05 補充:
我諗你係講緊(2 sin x-1)^2+5,
-1 <= sin x <= 1
-2 <= 2 sin x <= 2
-3 <= (2 sin x-1) <= 1
-3 <= (2 sin x-1) <= 0 or 0 <= (2sin x-1) <= 1
9 >= (2 sin x-1)^2 >= 0 or 0 <= (2 sin x-1)^2 <= 1
Therefore, 14 >= (2 sin x-1)^2+5 >= 5
所以最大值係 14,最細值係 5。