Cosx-Sinx=\(\sqrt{3}\)cos3x
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HP
1 tháng 6 2021
1.
\(sinx-\sqrt{2}cos3x=\sqrt{3}cosx+\sqrt{2}sin3x\)
\(\Leftrightarrow sinx-\sqrt{3}cosx=\sqrt{2}cos3x+\sqrt{2}sin3x\)
\(\Leftrightarrow\dfrac{1}{2}sinx-\dfrac{\sqrt{3}}{2}cosx=\dfrac{1}{\sqrt{2}}cos3x+\dfrac{1}{\sqrt{2}}sin3x\)
\(\Leftrightarrow sin\left(x-\dfrac{\pi}{3}\right)=sin\left(3x+\dfrac{\pi}{4}\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{\pi}{3}=3x+\dfrac{\pi}{4}+k2\pi\\x-\dfrac{\pi}{3}=\pi-3x-\dfrac{\pi}{4}+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{7\pi}{24}-k\pi\\x=-\dfrac{3}{4}x+\dfrac{13\pi}{48}+\dfrac{k\pi}{2}\end{matrix}\right.\)
Vậy phương trình đã cho có nghiệm \(x=-\dfrac{7\pi}{24}-k\pi;x=-\dfrac{3}{4}x+\dfrac{13\pi}{48}+\dfrac{k\pi}{2}\)
HP
1 tháng 6 2021
2.
\(sinx-\sqrt{3}cosx=2sin5\text{}x\)
\(\Leftrightarrow\dfrac{1}{2}sinx-\dfrac{\sqrt{3}}{2}cosx=sin5x\)
\(\Leftrightarrow sin\left(x-\dfrac{\pi}{3}\right)=sin5x\)
\(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{\pi}{3}=5x+k2\pi\\x-\dfrac{\pi}{3}=\pi-5x+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{12}-\dfrac{k\pi}{2}\\x=\dfrac{2\pi}{9}+\dfrac{k\pi}{3}\end{matrix}\right.\)
Vậy phương trình đã cho có nghiệm \(x=-\dfrac{\pi}{12}-\dfrac{k\pi}{2};x=\dfrac{2\pi}{9}+\dfrac{k\pi}{3}\)
21 tháng 7 2022
b: \(\Leftrightarrow2\cdot\cos2x\cdot\cos x+2\cdot\sin x\cdot\cos2x=\sqrt{2}\cdot\cos2x\)
\(\Leftrightarrow2\cdot\cos2x\left(\sin x+\cos x\right)=\sqrt{2}\cdot\cos2x\)
\(\Leftrightarrow\sqrt{2}\cdot\cos2x\cdot\left[\sqrt{2}\cdot\sqrt{2}\cdot\sin\left(x+\dfrac{\Pi}{4}\right)-1\right]=0\)
\(\Leftrightarrow\left[{}\begin{matrix}\cos2x=0\\\sin\left(x+\dfrac{\Pi}{4}\right)=\dfrac{1}{2}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}2x=\dfrac{\Pi}{2}+k\Pi\\x+\dfrac{\Pi}{4}=\dfrac{\Pi}{6}+k2\Pi\\x+\dfrac{\Pi}{4}=\dfrac{5}{6}\Pi+k2\Pi\end{matrix}\right.\)
\(\Leftrightarrow x\in\left\{\dfrac{\Pi}{4}+\dfrac{k\Pi}{2};\dfrac{-1}{12}\Pi+k2\Pi;\dfrac{7}{12}\Pi+k2\Pi\right\}\)
c: \(\Leftrightarrow2\cdot\sin2x\cdot\cos x+\sin2x=2\cdot\cos2x\cdot\cos x+\cos2x\)
\(\Leftrightarrow\sin2x\left(2\cos x+1\right)=\cos2x\left(2\cos x+1\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}\sin2x=\cos2x=\sin\left(\dfrac{\Pi}{2}-2x\right)\\\cos x=-\dfrac{1}{2}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\Pi}{8}+\dfrac{k\Pi}{4}\\\\x=-\dfrac{2}{3}\Pi+k2\Pi\\x=\dfrac{2}{3}\Pi+k2\Pi\end{matrix}\right.\)
NV
Nguyễn Việt Lâm
Giáo viên
30 tháng 7 2021
\(\Leftrightarrow cos3x+\sqrt{3}sin3x=\sqrt{3}cosx+sinx\)
\(\Leftrightarrow\dfrac{1}{2}cos3x+\dfrac{\sqrt{3}}{2}sin3x=\dfrac{\sqrt{3}}{2}cosx+\dfrac{1}{2}sinx\)
\(\Leftrightarrow cos\left(3x-\dfrac{\pi}{3}\right)=cos\left(x-\dfrac{\pi}{6}\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}3x-\dfrac{\pi}{3}=x-\dfrac{\pi}{6}+k2\pi\\3x-\dfrac{\pi}{3}=\dfrac{\pi}{6}-x+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{12}+k\pi\\x=\dfrac{\pi}{8}+\dfrac{k\pi}{2}\end{matrix}\right.\)
12 tháng 7 2016
<=> 2.cos2x.cosx = \(2\sqrt{3}\) cos2x.sinx
<=> cos2x.cosx = \(\sqrt{3}\) cos2x.sinx
<=> cos2x.( cosx - \(\sqrt{3}\) sinx) = 0
<=> \(\left[\begin{array}{nghiempt}cos2x=0\\cosx-\sqrt{3}sinx=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}2x=\frac{\pi}{2}+k\pi\\\frac{1}{2}cosx-\frac{\sqrt{3}}{2}sinx=0
\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\sin\frac{\pi}{6}.cosx-cos\frac{\pi}{6}.sinx=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\sin\left(\frac{\pi}{6}-x\right)=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\x=\frac{\pi}{6}-k\pi\end{array}\right.\) (k\(\in\)Z)
NV
Nguyễn Việt Lâm
Giáo viên
26 tháng 7 2020
c/
\(\Leftrightarrow\frac{1}{2}cosx-\frac{\sqrt{3}}{2}sinx=cos3x\)
\(\Leftrightarrow cos\left(x+\frac{\pi}{3}\right)=cos3x\)
\(\Rightarrow\left[{}\begin{matrix}x+\frac{\pi}{3}=3x+k2\pi\\x+\frac{\pi}{3}=-3x+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{6}+k\pi\\x=\frac{\pi}{12}+\frac{k\pi}{2}\end{matrix}\right.\)
d/
\(\Leftrightarrow\frac{1}{2}sin3x-\frac{\sqrt{3}}{2}cos3x=sin2x\)
\(\Leftrightarrow sin\left(3x-\frac{\pi}{3}\right)=sin2x\)
\(\Leftrightarrow\left[{}\begin{matrix}3x-\frac{\pi}{3}=2x+k2\pi\\3x-\frac{\pi}{3}=\pi-2x+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{3}+k2\pi\\x=\frac{4\pi}{15}+\frac{k2\pi}{5}\end{matrix}\right.\)
NV
Nguyễn Việt Lâm
Giáo viên
26 tháng 7 2020
a/
\(\Leftrightarrow\frac{1}{2}sinx+\frac{\sqrt{3}}{2}cosx=sin\left(x+\frac{\pi}{6}\right)\)
\(\Leftrightarrow sin\left(x+\frac{\pi}{3}\right)=sin\left(x+\frac{\pi}{6}\right)\)
\(\Rightarrow x+\frac{\pi}{3}=\pi-x-\frac{\pi}{6}+k2\pi\)
\(\Rightarrow x=\frac{\pi}{4}+k\pi\)
b/
\(\Leftrightarrow\frac{\sqrt{3}}{2}sinx+\frac{1}{2}cosx=sin\frac{\pi}{12}\)
\(\Leftrightarrow sin\left(x+\frac{\pi}{6}\right)=sin\frac{\pi}{12}\)
\(\Rightarrow\left[{}\begin{matrix}x+\frac{\pi}{6}=\frac{\pi}{12}+k2\pi\\x+\frac{\pi}{6}=\frac{11\pi}{12}+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-\frac{\pi}{12}+k2\pi\\x=\frac{3\pi}{4}+k2\pi\end{matrix}\right.\)
HP
22 tháng 8 2021
1.
Hàm số xác định khi \(\left\{{}\begin{matrix}\dfrac{1+x}{1-x}\ge0\\1-x\ne0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-1\le x< 1\\x\ne1\end{matrix}\right.\Leftrightarrow-1\le x< 1\)
2.
Hàm số xác định khi \(cosx+1\ne0\Leftrightarrow cosx\ne-1\Leftrightarrow x\ne-\pi+k2\pi\)
3.
Hàm số xác định khi \(cosx-cos3x\ne0\Leftrightarrow sin2x.sinx\ne0\Leftrightarrow\left[{}\begin{matrix}x\ne k\pi\\x\ne\dfrac{k\pi}{2}\end{matrix}\right.\)
NV
Nguyễn Việt Lâm
Giáo viên
2 tháng 4 2020
\(sin3x-cos3x=\left(3sinx-4sin^3x\right)-\left(4cos^3x-3cosx\right)\)
\(=3\left(sinx+cosx\right)-4\left(sin^3x+cos^3x\right)\)
\(=2\left(sin^3x+cos^3x\right)-6\left(sin^3x+cos^3x\right)+3\left(sinx+cosx\right)\)
\(=2\left(sin^3x+cos^3x\right)-6\left(sinx+cosx\right)\left(1-sinx.cosx\right)+3\left(sinx+cosx\right)\)
\(=2\left(sin^3x+cos^3x\right)-3\left(sinx+cosx\right)\left(1-2sinx.cosx\right)\)
\(=2\left(sin^3x+cos^3x\right)+6sinx.cosx\left(sinx+cosx\right)-3\left(sinx+cosx\right)\)
\(=2\left(sinx+cosx\right)^3-3\left(sinx+cosx\right)\) (đpcm)
NV
Nguyễn Việt Lâm
Giáo viên
1 tháng 8 2021
ĐKXĐ: ...
\(sin3x-cos3x+sinx+cosx=\dfrac{sin3x-cos3x+sinx+cosx}{\left(sin3x+cosx\right)\left(cos3x-sinx\right)}\)
\(\Rightarrow\left[{}\begin{matrix}sin3x-cos3x+sinx+cosx=0\left(1\right)\\\left(sin3x+cosx\right)\left(cos3x-sinx\right)=1\left(2\right)\end{matrix}\right.\)
(1) \(\Leftrightarrow3sinx-4sin^3x-4cos^3x+3cosx+sinx+cosx=0\)
\(\Leftrightarrow sinx+cosx+sin^3x+cos^3x=0\)
\(\Leftrightarrow sinx+cosx+\left(sinx+cosx\right)\left(1-sinx.cosx\right)=0\)
\(\Leftrightarrow\left(sinx+cosx\right)\left(2-sinx.cosx\right)=0\)
\(\Leftrightarrow sinx+cosx=0\) (loại)
(2) \(\Leftrightarrow sin3x.cos3x-sinx.cosx-sin3x.sinx+cos3x.cosx=1\)
\(\Leftrightarrow\dfrac{1}{2}sin6x-\dfrac{1}{2}sin2x+cos4x=1\)
\(\Leftrightarrow\dfrac{1}{2}\left(3sin2x-4sin^32x\right)-\dfrac{1}{2}sin2x+1-2sin^22x=1\)
\(\Leftrightarrow sin2x-2sin^32x-2sin^22x=0\)
\(\Leftrightarrow-sin2x\left(2sin^22x+2sin2x-1\right)=0\)
\(\Leftrightarrow...\)
D
1
22 tháng 10 2017
\(sinx+cosx\cdot sin2x+\sqrt{3}cos3x=2.\left(cos4x+sin^3x\right)\)
\(\Leftrightarrow sinx+cosx\cdot sin2x+\sqrt{3}cos3x=2cos4x+2sin^3x\)
\(\Leftrightarrow sinx-2sin^3x+cosx.sin2x+\sqrt{3}cos3x=2cos4x\)
\(\Leftrightarrow sinx.\left(1-2sin^2x\right)+cosx.sin2x+\sqrt{3}cos3x=2cos4x\)
\(\Leftrightarrow sinx.cos2x+cosx.sin2x+\sqrt{3}cos3x=2cos4x\)
\(\Leftrightarrow sin.\left(x+2x\right)+\sqrt{3}cos3x=2cos4x\)
\(\Leftrightarrow sin3x+\sqrt{3}cos3x=2cos4x\)
\(\Leftrightarrow\dfrac{1}{2}sin3x+\dfrac{\sqrt{3}}{2}cos3x=cos4x\)
\(\Leftrightarrow cos\dfrac{\pi}{3}.sin3x+sin\dfrac{\pi}{3}.cos3x=cos4x\)
\(\Leftrightarrow sin.\left(3x+\dfrac{\pi}{3}\right)=sin\left(\dfrac{\pi}{x}-4x\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}3x+\dfrac{\pi}{3}=\dfrac{\pi}{2}-4x+k2\pi\\3x+\dfrac{\pi}{2}=\pi-\dfrac{\pi}{2}+4x+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{42}+\dfrac{k2\pi}{7}\\x=-\dfrac{\pi}{6}+k2\pi\end{matrix}\right.\left(k\in Z\right)\)