問數(關於不等式同NO OF ROOTS)

以下是參考題解

1 -10 < 3x + 5 < -1 or 5x + 6 < -14

-15 < 3x < -6 or 5x < -20

-5 < x < -2 or x < -4

畫數線後可以知道答案是x < -2

2 y = 2cos^2 x = 5sin x + 4

2cos^2 x = 5sin x + 4

2(1 - sin^2 x) = 5sin x + 4

2sin^2 x + 5sin x + 2 = 0

(2sin x + 1)(sin x + 2) = 0

sin x = -1/2 or sin x = -2 (捨去)

因此有2個交點