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\(cos3x=-cos\left(x-120^0\right)\)
\(\Leftrightarrow cos3x=cos\left(x+60^0\right)\)
\(\Rightarrow\left[{}\begin{matrix}3x=x+60^0+k360^0\\3x=-x-60^0+k360^0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=30^0+k180^0\\x=-15^0+k90^0\end{matrix}\right.\)
\(\Leftrightarrow sin\left(2x-90^0\right)=cos2x\)
\(\Leftrightarrow-cos2x=cos2x\)
\(\Rightarrow cos2x=0\Rightarrow2x=90^0+k180^0\)
\(\Rightarrow x=45^0+k90^0\)
\(cos^2x+sin^2x+2sinx.cosx=1+cos4x\)
\(\Leftrightarrow1+sin2x=1+cos4x\)
\(\Leftrightarrow cos4x=sin2x=cos\left(\frac{\pi}{2}-2x\right)\)
\(\Rightarrow\left[{}\begin{matrix}4x=\frac{\pi}{2}-2x+k2\pi\\4x=2x-\frac{\pi}{2}+k2\pi\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{12}+\frac{k\pi}{3}\\x=-\frac{\pi}{4}+k\pi\end{matrix}\right.\)
a/ \(4cos^3x-3cosx-4\left(2cos^2x-1\right)+3cosx-4=0\)
\(\Leftrightarrow4cos^3x-8cos^2x=0\)
\(\Leftrightarrow4cos^2x\left(cosx-2\right)=0\)
\(\Leftrightarrow cosx=0\Rightarrow x=\frac{\pi}{2}+k\pi\)
\(0< \frac{\pi}{2}+k\pi< 14\Rightarrow-\frac{1}{2}< k< \frac{14-\frac{\pi}{2}}{\pi}\Rightarrow k=\left\{0;1;2;3\right\}\)
\(\Rightarrow x=\left\{\frac{\pi}{2};\frac{3\pi}{2};\frac{5\pi}{2};\frac{7\pi}{2}\right\}\)
b/ Bạn coi lại đề, cái ngoặc thứ 2 thiếu \(\left(2cos\left(???\right)+cosx\right)\)
c/ Bạn coi lại đề, có 2 số hạng \(cos2x\) xuất hiện ở vế trái, cấp 3 chắc ko ai cho kiểu vậy đâu, nếu đúng thế thì người ta cộng luôn thành \(2cos2x\) cho rồi
\(cosx+cos3x+cos2x+cos4x=0\)
\(\Leftrightarrow2cos2x.cosx+2cos3x.cosx=0\)
\(\Leftrightarrow cosx.\left(cos2x+cos3x\right)=0\)
\(\Leftrightarrow cosx.cos\frac{5x}{2}.cos\frac{x}{2}=0\)
\(\Rightarrow\left[{}\begin{matrix}cosx=0\\cos\frac{5x}{2}=0\\cos\frac{x}{2}=0\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{2}+k\pi\\\frac{5x}{2}=\frac{\pi}{2}+k\pi\\\frac{x}{2}=\frac{\pi}{2}+k\pi\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{2}+k\pi\\x=\frac{\pi}{5}+\frac{k2\pi}{5}\\x=\pi+k2\pi\end{matrix}\right.\)
\(sinx+sin7x+sin3x+sin5x=0\)
\(\Leftrightarrow2sin4x.cos3x+2sin4x.cosx=0\)
\(\Leftrightarrow sin4x\left(cos3x+cosx\right)=0\)
\(\Leftrightarrow sin4x.cos2x.cosx=0\)
\(\Leftrightarrow sin4x=0\)
\(\Rightarrow4x=k\pi\Rightarrow x=\frac{k\pi}{4}\)
Lý do chỉ cần 1 pt sin4x=0 do sin4x bao hàm cả cosx và cos2x ở trong đó
sin 2 x - cos 2 x = cos 4 x ⇔ - cos 2 x = cos 4 x ⇔ 2 cos 3 x . cos x = 0
\(\Leftrightarrow2cos4x.cos2x+cos4x=\frac{1}{2}cos2x\left(cos4x+cos2x\right)+2\)
\(\Leftrightarrow3cos4x.cos2x+2cos4x=cos^22x+4\)
\(\Leftrightarrow3cos2x\left(2cos^22x-1\right)+2\left(2cos^22x-1\right)=cos^22x+4\)
\(\Leftrightarrow2cos^22x+cos^22x-cos2x-2=0\)
\(\Leftrightarrow\left(cos2x-1\right)\left(2cos^22x+3cos2x+2\right)=0\)
a: \(\sqrt{3^2+2^2}=\sqrt{13}\)
Chia hai vế cho căn 13, ta được:
\(\dfrac{3}{\sqrt{13}}\cdot\sin2x+\dfrac{2}{\sqrt{13}}\cdot\cos2x=\dfrac{3}{\sqrt{13}}\)
Đặt \(\cos a=\dfrac{3}{\sqrt{13}}\)
Ta được phương trình: \(\sin\left(2x+a\right)=\cos a=\sin\left(\dfrac{\Pi}{2}-a\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}2x+a=\dfrac{\Pi}{2}-a+k2\Pi\\2x+a=\dfrac{\Pi}{2}+a+k2\Pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{1}{2}\left(\dfrac{\Pi}{2}-2a+k2\Pi\right)\\x=\dfrac{\Pi}{4}+k\Pi\end{matrix}\right.\)
b: \(\Leftrightarrow cos^2x-sin^2x+cosx-sinx=0\)
\(\Leftrightarrow\left(cosx-sinx\right)\left(cosx+sinx+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}\cos x=\cos\left(\dfrac{\Pi}{2}-x\right)\\\sin\left(x-\dfrac{\Pi}{4}\right)=-1\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\Pi}{2}-x+k2\Pi\\x=-\dfrac{\Pi}{2}+x+k2\Pi\\x-\dfrac{\Pi}{4}=-\dfrac{\Pi}{2}+k2\Pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\Pi}{4}+k\Pi\\x=-\dfrac{\Pi}{4}+k2\Pi\end{matrix}\right.\)
Đkxđ: \(x\in R\).
\(cos2x-cos3x+cos4x=0\Leftrightarrow\left(cos2x+cos4x\right)-cos3x=0\)
\(\Leftrightarrow2cos3x.cosx-cos3x=0\)
\(\Leftrightarrow cos3x\left(2cos2x-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}cos3x=0\\2cos2x-1=0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}cos3x=0\\cos2x=\dfrac{1}{2}\end{matrix}\right.\)
\(cos3x=0\Leftrightarrow3x=\dfrac{\pi}{2}+k\pi\Leftrightarrow x=\dfrac{\pi}{6}+\dfrac{k\pi}{3}\)
\(cos2x=\dfrac{1}{2}\Leftrightarrow\left[{}\begin{matrix}2x=\dfrac{\pi}{3}+k2\pi\\2x=-\dfrac{\pi}{3}+k2\pi\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k\pi\\x=-\dfrac{\pi}{6}+k\pi\end{matrix}\right.\)
\(\dfrac{sinB}{sinC}=2cosA\Leftrightarrow sinB=2cosA.sinC\)
\(\Leftrightarrow sinB=sin\left(A+C\right)+sin\left(C-A\right)\)
\(\Leftrightarrow sinB=sin\left(\pi-\left(A+C\right)\right)+sin\left(C-A\right)\)
\(\Leftrightarrow sinB=sinB+sin\left(C-A\right)\)
\(\Leftrightarrow sin\left(C-A\right)=0\) (1)
Do A, C là số đo các góc trong tam giác nên từ (1) suy ra:
\(C=A\) hay tam giác ABC cân.
Toán tui ngu lắm, mấy tháng rồi chưa sờ đến toán luôn :) Nhờ tui làm chi
a/ \(2\cos^24x=1+\cos x.\cos3x-\sin x.\sin3x\)
\(\Leftrightarrow2\cos^24x=1+\cos\left(3x+x\right)=1+\cos4x\)
\(\Leftrightarrow2\cos^24x-\cos4x-1=0\)
Pt bậc 2, bạn tự giải
b/ \(\Leftrightarrow2.\frac{1}{2}\left[\cos2x-\cos8x\right]+\cos4x=\cos2x\)
\(\cos8x=\cos2.4x=2\cos^24x-1\)
\(\Rightarrow-2\cos^24x+1+\cos4x=0\)
\(\Leftrightarrow2\cos^24x-\cos4x-1=0\)
Pt bậc 2, bạn tự giải