1.

ĐK: \(x\ne\dfrac{\pi}{4}+k\pi\)

\(\dfrac{cos2x}{1-sin2x}=0\)

\(\Leftrightarrow cos2x=0\)

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

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

Đối chiếu điều kiên ta được \(x=-\dfrac{\pi}{4}+k\pi\)

2.

ĐK: \(x\ne\dfrac{\pi}{6}+\dfrac{k\pi}{3};x\ne\dfrac{\pi}{8}+\dfrac{k\pi}{4}\)

\(tan3x=tan4x\)

\(\Leftrightarrow3x=4x+k\pi\)

\(\Leftrightarrow x=k\pi\)

3.

ĐK: \(x\ne\dfrac{k\pi}{2}\)

\(cot2x.sin3x=0\)

\(\Leftrightarrow\dfrac{cos2x}{sin2x}.sin3x=0\)

\(\Leftrightarrow cos2x.sin3x=0\)

\(\Leftrightarrow\left[{}\begin{matrix}cos2x=0\\sin3x=0\end{matrix}\right.\)

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

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

Chọn A

\(sin2x-cos2x+3sinx-cosx-1=0\)

\(\Leftrightarrow2sinxcosx-\left(1-2sin^2x\right)+3sinx-cosx-1=0\)

\(\Leftrightarrow2sinxcosx-1+2sin^2x+3sinx-cosx-1=0\)

\(\Leftrightarrow2sin^2x+3sinx-2+cosx\left(2sinx-1\right)=0\)

\(\Leftrightarrow2\left(sinx-\dfrac{1}{2}\right)\left(sinx+2\right)+cosx\left(2sinx-1\right)=0\)

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

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

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

\(\Leftrightarrow\left[{}\begin{matrix}sinx=\dfrac{1}{2}\\sinx+cosx=-2\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}sinx=sin\dfrac{\pi}{6}\\\sqrt[]{2}\left(sinx.\dfrac{1}{\sqrt[]{2}}+cosx.\dfrac{1}{\sqrt[]{2}}\right)=-2\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k2\pi\\x=\dfrac{5\pi}{6}+k2\pi\\\sqrt[]{2}sin\left(x+\dfrac{\pi}{4}\right)=-2\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k2\pi\\x=\dfrac{5\pi}{6}+k2\pi\\sin\left(x+\dfrac{\pi}{4}\right)=-\sqrt[]{2}\left(vô.lý\right)\end{matrix}\right.\)

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

a) <=> 4sinxcosx -(2cos²x-1)=7sinx+2cosx-4

<=> 2cos²x+(2-4sinx)cosx+7sinx-5=0

- sinx=1 => 2cos²x-2cosx+2=0 

pt trên vn

b) <=> 2sinxcosx-1+2sin²x+3sinx-cosx-1=0

<=> cos(2sinx-1)+2sin²x+3sinx-2=0

<=> cosx(2sinx-1)+(2sinx-1)(sinx+2)=0

<=> (2sinx-1)(cosx+sinx+2)=0

<=> sinx=1/2 hoặc cosx+sinx=-2(vn)

<=> x= \(\frac{\pi}{6}+k2\pi\) hoặc \(x=\frac{5\pi}{6}+k2\pi\left(k\in Z\right)\)

<=> (2sinxcosx-cosx)+5sinx-2-cos2x=0

<=> cosx(2sinx-1)+2\(sin^2x\)+5sinx-3=0

<=> cosx(2sinx-1) +(2sinx-1)(sinx+3)

<=> (2sinx-1)(cosx+sinx+3)=0

<=>\(\begin{cases}sinx=\frac{1}{2}\\cosx+sinx+3=0\end{cases}\)

+) sinx=1/2

<=> \(x=\frac{\pi}{2}+k2\pi\) với k thuộc Z

+) cosx+sinx+3= <=>\(\sqrt{2}sin\left(x+\frac{\pi}{4}\right)\)=-3

<=> \(sin\left(x+\frac{\pi}{4}\right)\)=\(\frac{-\sqrt{3}}{2}\)

<=>\(sin\left(x+\frac{\pi}{4}\right)=sin\frac{-\pi}{3}\)

<=> \(\left[\begin{array}{nghiempt}x=\frac{-\pi}{3}+k2\pi\\x=\frac{4\pi}{3}+k2\pi\end{array}\right.\)với k thuộc Z

vậy pht có 3 nghiệm:..

a, \(sin4x.cosx-sin3x=0\)

\(\Leftrightarrow\dfrac{1}{2}sin5x+\dfrac{1}{2}sin3x-sin3x=0\)

\(\Leftrightarrow sin5x=sin3x\)

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

\(\Leftrightarrow\left[{}\begin{matrix}x=k\pi\\x=\dfrac{\pi}{8}+\dfrac{k\pi}{4}\end{matrix}\right.\)

b, \(sin2x+\sqrt{3}cos2x=\sqrt{2}\)

\(\Leftrightarrow\dfrac{1}{2}sin2x+\dfrac{\sqrt{3}}{2}cos2x=\dfrac{\sqrt{2}}{2}\)

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

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

\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{24}+k\pi\\x=\dfrac{5\pi}{24}+k\pi\end{matrix}\right.\)