`a) y = 4 - 2sin x`
Ta có:
`-1 ≤ sin x ≤ 1`
`<=> -2 ≤ -2sin x ≤ 2`
`<=> 2 ≤ y ≤ 6`
`b) y = 5 - 9cos x`
Ta có:
`-1 ≤ cos x ≤ 1`
`<=> -9 ≤ -9cos x ≤ 9`
`<=> -4 ≤ y ≤ 14`
`c) y = 4 - 2cos x`
Ta có:
`-1 ≤ cos x ≤ 1`
`<=> -2 ≤ -2cos x ≤ 2`
`<=> 2 ≤ y ≤ 6`
`d) y = 5 - 9cos x.sin x = 5 - 9/(2)sin 2x`
Ta có:
`-1 ≤ sin 2x ≤ 1`
`<=> -9/2 ≤ -9/(2)sin 2x ≤ 9/2`
`<=> 1/2 ≤ y ≤ 19/2`
`e) y = 3 - 7cos x`
Ta có:
`-1 ≤ cos x ≤ 1`
`<=> -7 ≤ -7cos x ≤ 7`
`<=> - 4 ≤ y ≤ 10`
