Chọn A.

Ta có:

sinx + cosx = ½ nên ( sinx + cosx)² = ¼

Do đó sinx. cosx = -3/8

Khi đó sinx; cosx là nghiệm của phương trình 

Ta có sinx + cos x = ½ nên 2( sinx + cosx) = 1

+) Với 

+) Với 

1.

\(2sin\left(x+\dfrac{\pi}{6}\right)+sinx+2cosx=3\)

\(\Leftrightarrow\sqrt{3}sinx+cosx+sinx+2cosx=3\)

\(\Leftrightarrow\left(\sqrt{3}+1\right)sinx+3cosx=3\)

\(\Leftrightarrow\sqrt{13+2\sqrt{3}}\left[\dfrac{\sqrt{3}+1}{\sqrt{13+2\sqrt{3}}}sinx+\dfrac{3}{\sqrt{13+2\sqrt{3}}}cosx\right]=3\)

Đặt \(\alpha=arcsin\dfrac{3}{\sqrt{13+2\sqrt{3}}}\)

\(pt\Leftrightarrow\sqrt{13+2\sqrt{3}}sin\left(x+\alpha\right)=3\)

\(\Leftrightarrow sin\left(x+\alpha\right)=\dfrac{3}{\sqrt{13+2\sqrt{3}}}\)

\(\Leftrightarrow\left[{}\begin{matrix}x+\alpha=arcsin\dfrac{3}{\sqrt{13+2\sqrt{3}}}+k2\pi\\x+\alpha=\pi-arcsin\dfrac{3}{\sqrt{13+2\sqrt{3}}}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=k2\pi\\x=\pi-2arcsin\dfrac{3}{\sqrt{13+2\sqrt{3}}}+k2\pi\end{matrix}\right.\)

Vậy phương trình đã cho có nghiệm:

\(x=k2\pi;x=\pi-2arcsin\dfrac{3}{\sqrt{13+2\sqrt{3}}}+k2\pi\)

2.

\(\left(sin2x+cos2x\right)cosx+2cos2x-sinx=0\)

\(\Leftrightarrow2sinx.cos^2x+cos2x.cosx+2cos2x-sinx=0\)

\(\Leftrightarrow\left(2cos^2x-1\right)sinx+cos2x.cosx+2cos2x=0\)

\(\Leftrightarrow cos2x.sinx+cos2x.cosx+2cos2x=0\)

\(\Leftrightarrow cos2x.\left(sinx+cosx+2\right)=0\)

\(\Leftrightarrow cos2x=0\)

\(\Leftrightarrow2x=\dfrac{\pi}{2}+k\pi\)

\(\Leftrightarrow x=\dfrac{\pi}{4}+\dfrac{k\pi}{2}\)

Vậy phương trình đã cho có nghiệm \(x=\dfrac{\pi}{4}+\dfrac{k\pi}{2}\)

a.

\(\Leftrightarrow sin2x+cos2x=3sinx+cosx+2\)

\(\Leftrightarrow2sinx.cosx-3sinx+2cos^2x-cosx-3=0=0\)

\(\Leftrightarrow sinx\left(2cosx-3\right)+\left(cosx+1\right)\left(2cosx-3\right)=0\)

\(\Leftrightarrow\left(sinx+cosx+1\right)\left(2cosx-3\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sinx+cosx=-1\\2cosx-3=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}sin\left(x+\frac{\pi}{4}\right)=-\frac{\sqrt{2}}{2}\\cosx=\frac{3}{2}\left(vn\right)\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x+\frac{\pi}{4}=-\frac{\pi}{4}+k2\pi\\x+\frac{\pi}{4}=\frac{5\pi}{4}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow...\)

b.

\(\Leftrightarrow1+sinx+cosx+2sinx.cosx+2cos^2x-1=0\)

\(\Leftrightarrow sinx\left(2cosx+1\right)+cosx\left(2cosx+1\right)=0\)

\(\Leftrightarrow\left(sinx+cosx\right)\left(2cosx+1\right)=0\)

\(\Leftrightarrow\sqrt{2}sin\left(x+\frac{\pi}{4}\right)\left(2cosx+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sin\left(x+\frac{\pi}{4}\right)=0\\cosx=-\frac{1}{2}\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-\frac{\pi}{4}+k\pi\\x=\frac{2\pi}{3}+k2\pi\\x=-\frac{2\pi}{3}+k2\pi\end{matrix}\right.\)

c.

\(\Leftrightarrow\left(2cosx-1\right)\left(2sinx+cosx\right)=2sinx.cosx-sinx\)

\(\Leftrightarrow\left(2cosx-1\right)\left(2sinx+cosx\right)-sinx\left(2cosx-1\right)=0\)

\(\Leftrightarrow\left(2cosx-1\right)\left(2sinx+cosx-sinx\right)=0\)

\(\Leftrightarrow\left(2cosx-1\right)\left(sinx+cosx\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}2cosx-1=0\\sinx+cosx=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}cosx=\frac{1}{2}\\sin\left(x+\frac{\pi}{4}\right)=0\end{matrix}\right.\)

\(\Leftrightarrow...\)

a ) \(2cosx-3sinx+2=0\) 

\(\Leftrightarrow2cosx-3sinx=-2\)  

\(\Leftrightarrow\dfrac{2}{\sqrt{13}}cosx-\dfrac{3}{\sqrt{13}}sinx=-\dfrac{2}{\sqrt{13}}\) 

Thấy : \(\left(\dfrac{2}{\sqrt{13}}\right)^2+\left(\dfrac{-3}{\sqrt{13}}\right)^2=1\) nên tồn tại \(\alpha\) t/m : 

\(sin\alpha=\dfrac{2}{\sqrt{13}};cos\alpha=\dfrac{-3}{\sqrt{13}}\) . . Khi đó : \(sin\alpha.cosx+cos\alpha.sinx=\dfrac{-2}{\sqrt{13}}\)

\(\Leftrightarrow sin\left(\alpha+x\right)=\dfrac{-2}{\sqrt{13}}\) ( p/t cơ bản ) 

b ) \(\dfrac{1+sinx}{1+cosx}=\dfrac{1}{2}\) ( ĐK : \(cosx\ne-1\Leftrightarrow x\ne\left(2k+1\right)\pi\) ; ( k thuộc Z )  ) 

\(\Leftrightarrow2+2sinx=cosx+1\) \(\Leftrightarrow cosx-2sinx=1\) 

Làm giống như a )  

c ) \(cos\left(2x-15^o\right)+sin\left(2x-15^o\right)=-1\)  

Đặt \(t=2x-15^o\) ; ta có : \(cos t + sin t = -1\) 

\(\Leftrightarrow\sqrt{2}sin\left(t+\dfrac{\pi}{4}\right)=-1\) \(\Leftrightarrow sin\left(t+\dfrac{\pi}{4}\right)=sin\left(-\dfrac{\pi}{4}\right)\)  

Xong rồi bn làm tiếp ; chú ý đổi ra độ 

a/

\(\Leftrightarrow1-2\left(2cos^2x-1\right)-\sqrt{3}sinx+cosx=0\)

\(\Leftrightarrow3-4cos^2x+cosx-\sqrt{3}sinx=0\)

\(\Leftrightarrow\left(1-cosx\right)\left(4cosx+3\right)-\sqrt{3}sinx=0\)

\(\Leftrightarrow2sin^2\frac{x}{2}\left(4cosx+3\right)-2\sqrt{3}sin\frac{x}{2}cos\frac{x}{2}=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}sin\frac{x}{2}=0\Rightarrow x=k2\pi\\sin\frac{x}{2}\left(4cosx+3\right)-\sqrt{3}cos\frac{x}{2}=0\left(1\right)\end{matrix}\right.\)

Xét (1) \(\Leftrightarrow sin\frac{x}{2}\left(8cos^2\frac{x}{2}-1\right)-\sqrt{3}cos\frac{x}{2}=0\)

- Với \(\left\{{}\begin{matrix}cos\frac{x}{2}=0\\sin\frac{x}{2}=-1\end{matrix}\right.\) \(\Rightarrow x=-\pi+k4\pi\) là 1 nghiệm

- Với \(cos\frac{x}{2}\ne0\) chia 2 vế cho \(cos^3\frac{x}{2}\)

\(tan\frac{x}{2}\left(8-1-tan^2\frac{x}{2}\right)-\sqrt{3}-\sqrt{3}tan^2\frac{x}{2}=0\)

\(\Leftrightarrow-tan^3\frac{x}{2}-\sqrt{3}tan^2\frac{x}{2}+7tan\frac{x}{2}-\sqrt{3}=0\)

Đặt \(tan\frac{x}{2}=t\)

\(\Rightarrow t^3+\sqrt{3}t^2-7t+\sqrt{3}=0\)

\(\Leftrightarrow\left[{}\begin{matrix}t=\sqrt{3}\\t=-2-\sqrt{3}\\t=2-\sqrt{3}\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\frac{x}{2}=\frac{\pi}{3}+k\pi\\\frac{x}{2}=-\frac{5\pi}{12}+k\pi\\\frac{x}{2}=\frac{\pi}{12}+k\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=\frac{2\pi}{3}+k2\pi\\x=-\frac{5\pi}{6}+k2\pi\\x=\frac{\pi}{6}+k2\pi\end{matrix}\right.\)

b/

\(\Leftrightarrow cos^2x-sin^2x+cos^2x-sinx.cosx=8\left(cosx-sinx\right)\)

\(\Leftrightarrow\left(cosx-sinx\right)\left(cosx+sinx\right)+cosx\left(cosx-sinx\right)=8\left(cosx-sinx\right)\)

\(\Leftrightarrow\left(cosx-sinx\right)\left(2cosx+sinx-8\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}cosx-sinx=0\left(1\right)\\2cosx+sinx=8\left(2\right)\end{matrix}\right.\)

\(\left(1\right)\Leftrightarrow sin\left(x-\frac{\pi}{4}\right)=0\Leftrightarrow x-\frac{\pi}{4}=k\pi\)

\(\Rightarrow x=\frac{\pi}{4}+k\pi\)

Xét (2), theo điều kiện có nghiệm của pt lượng giác bậc nhất, \(2^2+1^2< 8^2\Rightarrow\left(2\right)\) vô nghiệm

c/

\(\Leftrightarrow\left(sinx-cosx\right)\left(sinx+4cosx\right)=4\left(sinx-cosx\right)\)

\(\Leftrightarrow\left(sinx-cosx\right)\left(sinx+4cosx-4\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sinx-cosx=0\left(1\right)\\sinx+4cosx-4=0\left(2\right)\end{matrix}\right.\)

Xét (1) \(\Leftrightarrow sin\left(x-\frac{\pi}{4}\right)=0\Leftrightarrow x=\frac{\pi}{4}+k\pi\)

Xét (2) \(\Leftrightarrow\frac{1}{\sqrt{17}}sinx+\frac{4}{\sqrt{17}}cosx=\frac{4}{\sqrt{17}}\)

Đặt \(\frac{4}{\sqrt{17}}=cosa\) với \(a\in\left(0;\pi\right)\)

\(\Rightarrow cosx.cosa+sinx.sina=cosa\)

\(\Leftrightarrow cos\left(x-a\right)=cosa\)

\(\Leftrightarrow\left[{}\begin{matrix}x-a=a+k2\pi\\x-a=-a+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=2a+k2\pi\\x=k2\pi\end{matrix}\right.\)

2.1

a.

\(\Leftrightarrow sinx-cosx=\dfrac{\sqrt{2}}{2}\)

\(\Leftrightarrow\sqrt{2}sin\left(x-\dfrac{\pi}{4}\right)=\dfrac{\sqrt{2}}{2}\)

\(\Leftrightarrow sin\left(x-\dfrac{\pi}{4}\right)=\dfrac{1}{2}\)

\(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{\pi}{4}=\dfrac{\pi}{6}+k2\pi\\x-\dfrac{\pi}{4}=\dfrac{5\pi}{6}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{5\pi}{12}+k2\pi\\x=\dfrac{13\pi}{12}+k2\pi\end{matrix}\right.\)

b.

\(cosx-\sqrt{3}sinx=1\)

\(\Leftrightarrow\dfrac{1}{2}cosx-\dfrac{\sqrt{3}}{2}sinx=\dfrac{1}{2}\)

\(\Leftrightarrow cos\left(x+\dfrac{\pi}{3}\right)=\dfrac{1}{2}\)

\(\Leftrightarrow\left[{}\begin{matrix}x+\dfrac{\pi}{3}=\dfrac{\pi}{3}+k2\pi\\x+\dfrac{\pi}{3}=-\dfrac{\pi}{3}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=k2\pi\\x=-\dfrac{2\pi}{3}+k2\pi\end{matrix}\right.\)

c.

\(\sqrt{3}sin\dfrac{x}{3}+cos\dfrac{x}{2}=\sqrt{2}\)

Câu này đề đúng không nhỉ? Nhìn thấy có vẻ không đúng lắm

d.

\(cosx-sinx=1\)

\(\Leftrightarrow\sqrt{2}cos\left(x+\dfrac{\pi}{4}\right)=1\)

\(\Leftrightarrow cos\left(x+\dfrac{\pi}{4}\right)=\dfrac{\sqrt{2}}{2}\)

\(\Leftrightarrow\left[{}\begin{matrix}x+\dfrac{\pi}{4}=\dfrac{\pi}{4}+k2\pi\\x+\dfrac{\pi}{4}=-\dfrac{\pi}{4}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=k2\pi\\x=-\dfrac{\pi}{2}+k2\pi\end{matrix}\right.\)

Đk: \(\left\{{}\begin{matrix}x\ne\dfrac{\pi}{2}+m2\pi\\x\ne\dfrac{\pi}{4}+n\pi\end{matrix}\right.\left(m,n\in Z\right)\)

PT \(\Leftrightarrow1=2\sqrt{2}sinx.cosx\left(sinx-cosx\right)+2cos^2x\)

\(\Leftrightarrow\sqrt{2}.2sinx.cosx\left(sinx-cosx\right)+\left(2cos^2x-1\right)=0\)

\(\Leftrightarrow\sqrt{2}sin2x\left(sinx-cosx\right)+\left(cosx-sinx\right)\left(cosx+sinx\right)=0\)

\(\Leftrightarrow\sqrt{2}sin2x=sinx+cosx\)

\(\Leftrightarrow\sqrt{2}sin2x=\sqrt{2}sin\left(x+\dfrac{\pi}{4}\right)\)

\(\Leftrightarrow\left[{}\begin{matrix}2x=x+\dfrac{\pi}{4}+k2\pi\\2x=\pi-x-\dfrac{\pi}{4}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{4}+k2\pi\\x=\dfrac{\pi}{4}+k\dfrac{2\pi}{3}\end{matrix}\right.\left(k\in Z\right)\)