`~rai~`
\(1.y=3\sin^22x-4\\\text{Ta có:}0\le\sin^22x\le 1\\\Leftrightarrow 0\le3\sin^22x\le 3\\\Leftrightarrow -4\le3\sin^22x-4\le -1\\\Leftrightarrow -4\le y\le1.\\+)Min_y=-4\Leftrightarrow \sin^22x=0\\\Leftrightarrow \sin2x=0\\\Leftrightarrow 2x=k\pi\\\Leftrightarrow x=k\dfrac{\pi}{2}.(k\in\mathbb{Z})\\+)Max_y=-1\Leftrightarrow \sin^22x=1\\\Leftrightarrow \cos^22x=0\\\Leftrightarrow \cos2x=0\\\Leftrightarrow 2x=\dfrac{\pi}{2}+k\pi\\\Leftrightarrow x=\dfrac{\pi}{4}+k\dfrac{\pi}{2}.(k\in\mathbb{Z})\\\text{Vậy Min}_y=-4\text{ khi x=}k\dfrac{\pi}{2};\\\text{Max}_y=-1\text{ khi x=}\dfrac{\pi}{4}+k\dfrac{\pi}{2}.(k\in\mathbb{Z})\\2.y=1-\sin x\cos x\\\quad=\dfrac{1}{2}(2-2\sin x\cos x)\\\quad=\dfrac{1}{2}(2-\sin2x).\\\text{Ta có:}-1\le \sin2x\le 1\\\Leftrightarrow -1\le -\sin2x\le 1\\\Leftrightarrow 1\le2-\sin2x\le 3\\\Leftrightarrow \dfrac{1}{2}\le\dfrac{1}{2}(1-\sin2x)\le\dfrac{3}{2}\\\Leftrightarrow \dfrac{1}{2}\le y\le \dfrac{3}{2}.\\+)Min_y=\dfrac{1}{2}\Leftrightarrow \sin2x=1\\\Leftrightarrow 2x=\dfrac{\pi}{2}+k2\pi\\\Leftrightarrow x=\dfrac{\pi}{4}+k\pi.\\+)Max_y=\dfrac{3}{2}\Leftrightarrow \sin2x=-1\\\Leftrightarrow 2x=-\dfrac{\pi}{2}+k2\pi\\\Leftrightarrow x=-\dfrac{\pi}{4}+k\pi.(k\in\mathbb{Z})\\\text{Vậy Min}_y=\dfrac{1}{2}\text{ khi x=}\dfrac{\pi}{4}+k\pi;\\\text{Max}_y=\dfrac{3}{2}\text{ khi x=}-\dfrac{\pi}{4}+k\pi.(k\in\mathbb{Z})\)
