\(N=\frac{\frac{3sin^2x}{cos^2x}+\frac{12sinx.cosx}{cos^2x}+\frac{cos^2x}{cos^2x}}{\frac{sin^2x}{cos^2x}+\frac{sinx.cosx}{cos^2x}-\frac{2cos^2x}{cos^2x}}=\frac{3tan^2x+12tanx+1}{tan^2x+tanx-2}=...\)

1: Ta có: \(-1<=\sin\left(2x+\frac{\pi}{4}\right)\le1\)

=>\(-3\le3\cdot\sin\left(2x+\frac{\pi}{4}\right)\le3\)

=>\(-3-1\le3\cdot\sin\left(2x+\frac{\pi}{4}\right)-1\le3-1\)

=>-4<=y<=2

=>Tập giá trị là T=[-4;2]

\(y_{\min}=-4\) khi \(\sin\left(2x+\frac{\pi}{4}\right)=-1\)

=>\(2x+\frac{\pi}{4}=-\frac{\pi}{2}+k2\pi\)

=>\(2x=-\frac34\pi+k2\pi\)

=>\(x=-\frac38\pi+k\pi\)

2: \(0\le cos^2x\le1\)

=>\(0\ge-5\cdot cos^2x\ge-5\)

=>\(0+3\ge-5\cdot cos^2x+3\ge-5+3\)

=>3>=y>=-2

=>Tập giá trị là T=[-2;3]

\(y_{\max}=3\) khi \(cos^2x=1\)

=>\(\sin^2x=0\)

=>sin x=0

=>\(x=k\pi\)

\(y_{\min}=-2\) khi \(cos^2x=0\)

=>cosx=0

=>\(x=\frac{k\pi}{2}\)

3: \(-1\le cosx\le1\)

=>\(-3\le3\cdot cosx\le3\)

=>\(-3+4\le3\cdot cosx+4\le3+4\)

=>\(1\le3\cdot cosx+4\le7\)

=>\(\frac51\ge\frac{5}{3\cdot cosx+4}\ge\frac57\)

=>\(\frac57\le y\le5\)

=>Tập giá trị là \(T=\left\lbrack\frac57;5\right\rbrack\)

\(y_{\min}=\frac57\) khi cosx=1

=>\(x=k2\pi\)

\(y_{\max}=5\) khi cosx=-1

=>\(x=\pi+k2\pi\)

4: \(y=\sin^2x-4\cdot\sin x+8\)

\(=\sin^2x-4\cdot\sin x+4+4\)

\(=\left(\sin x-2\right)^2+4\)

Ta có: \(-1\le\sin x\le1\)

=>\(-1-2\le\sin x-2\le1-2\)

=>\(-3\le\sin x-2\le-1\)

=>\(1\le\left(\sin x-2\right)^2\le9\)

=>\(5\le\left(\sin x-2\right)^2+4\le13\)

=>5<=y<=13

=>Tập giá trị là T=[5;13]

\(y_{\min}=5\) khi sin x-2=-1

=>sin x=1

=>\(x=\frac{\pi}{2}+k2\pi\)

\(y_{\max}\) =13 khi sin x-2=-3

=>sin x=-1

=>\(x=-\frac{\pi}{2}+k2\pi\)

Biến đổi tớ gọi B nhá cậu :)
\(B=\frac{\frac{3}{x}}{5-\frac{3}{2x}}=\frac{4x+3}{x}:\frac{10x-3}{2x}=\frac{4x+3}{x}.\frac{2x}{10x-3}=\frac{8x+6}{10x-3}\)

Bài này đơn giản làm theo dạng là được =))

Mình vội nên suy nghĩ có 5 phút nếu sai sót gì mong bạn thông cảm

d.

Nhận thấy \(cosx=0\) ko phải nghiệm, chia 2 vế cho \(cos^4x\)

\(tan^4x-3tan^2x-4tanx-3=0\)

\(\Leftrightarrow\left(tan^2x+tanx+1\right)\left(tan^2x-tanx-3\right)=0\)

\(\Leftrightarrow tan^2x-tanx-3=0\)

\(\Leftrightarrow\left[{}\begin{matrix}tanx=\frac{1-\sqrt{13}}{2}\\tanx=\frac{1+\sqrt{13}}{2}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=arctan\left(\frac{1-\sqrt{13}}{2}\right)+k\pi\\x=arctan\left(\frac{1+\sqrt{13}}{2}\right)+k\pi\end{matrix}\right.\)

mọi người giúp hộ mình nhanh với

\(B=cos^2x.cot^2x+cos^2x-cot^2x+2\left(sin^2x+cos^2x\right)\)

\(=cos^2x\left(cot^2x+1\right)-cot^2x+2\)

\(=\frac{cos^2x}{sin^2x}-cot^2x+1=cot^2x-cot^2x+1=1\)

\(M=cos^4x-sin^4x+cos^4x+sin^2x.cos^2x+3sin^2x\)

\(=\left(cos^2x-sin^2x\right)\left(cos^2x+sin^2x\right)+cos^2x\left(cos^2x+sin^2x\right)+3sin^2x\)

\(=cos^2x-sin^2x+cos^2x+3sin^2x\)

\(=2\left(sin^2x+cos^2x\right)=2\)

- Xét \(sin\frac{x}{5}=0\Rightarrow C=...\)

- Với \(sin\frac{x}{5}\ne0\)

\(C.sin\frac{x}{5}=sin\frac{x}{5}.cos\frac{x}{5}.cos\frac{2x}{5}cos\frac{4x}{5}cos\frac{8x}{5}\)

\(=\frac{1}{2}sin\frac{2x}{5}cos\frac{2x}{5}cos\frac{4x}{5}cos\frac{8x}{5}\)

\(=\frac{1}{4}sin\frac{4x}{5}cos\frac{4x}{5}cos\frac{8x}{5}=\frac{1}{8}sin\frac{8x}{5}cos\frac{8x}{5}\)

\(=\frac{1}{16}sin\frac{16x}{5}\Rightarrow C=\frac{sin\frac{16x}{5}}{16.sin\frac{x}{5}}\)

\(D=sin\frac{x}{7}+sin\frac{5x}{7}+2sin\frac{3x}{7}\)

\(=2sin\frac{3x}{7}cos\frac{2x}{7}+2sin\frac{3x}{7}\)

\(=2sin\frac{3x}{7}\left(cos\frac{2x}{7}+1\right)=4cos^2\frac{x}{7}.sin\frac{3x}{7}\)

\(A=cos\frac{\pi}{7}cos\frac{3\pi}{7}cos\frac{5\pi}{7}=cos\frac{\pi}{7}cos\frac{4\pi}{7}cos\frac{2\pi}{7}\)

\(\Rightarrow A.sin\frac{\pi}{7}=sin\frac{\pi}{7}.cos\frac{\pi}{7}.cos\frac{2\pi}{7}cos\frac{4\pi}{7}\)

\(=\frac{1}{2}sin\frac{2\pi}{7}cos\frac{2\pi}{7}cos\frac{4\pi}{7}=\frac{1}{4}sin\frac{4\pi}{7}cos\frac{4\pi}{7}\)

\(=\frac{1}{8}sin\frac{8\pi}{7}=\frac{1}{8}sin\left(\pi+\frac{\pi}{7}\right)=-\frac{1}{8}sin\frac{\pi}{7}\)

\(\Rightarrow A=-\frac{1}{8}\)

\(B=sin6.cos48.cos24.cos12\)

\(B.cos6=sin6.cos6.cos12.cos24.cos48\)

\(=\frac{1}{2}sin12.cos12.cos24.cos48=\frac{1}{4}sin24.cos24.cos48\)

\(=\frac{1}{8}sin48.cos48=\frac{1}{16}sin96\)

\(=\frac{1}{16}sin\left(90+6\right)=\frac{1}{16}cos6\Rightarrow B=\frac{1}{16}\)

1,\(A=3\left(sin^4x+cos^4x\right)-2\left(sin^2x+cos^2x\right)\left(sin^4x-sin^2x.cos^2x+cos^4x\right)\)

\(=3\left(sin^4x+cos^4x\right)-2\left(sin^4x-sin^2x.cos^4x+cos^4x\right)\)

\(=sin^4x+2sin^2x.cos^2x+cos^4x=\left(sin^2x+cos^2x\right)^2=1\)

Vậy...

2,\(B=cos^6x+2sin^4x\left(1-sin^2x\right)+3\left(1-cos^2x\right)cos^4x+sin^4x\)

\(=-2cos^6x+3sin^4x-2sin^6x+3cos^4x\)

\(=-2\left(sin^2x+cos^2x\right)\left(sin^4x-sin^2x.cos^2x+cos^4x\right)+3\left(cos^4x+sin^4x\right)\)

\(=-2\left(sin^4x-sin^2x.cos^2x+cos^4x\right)+3\left(cos^4x+sin^4x\right)\)\(=cos^4x+sin^4x+2sin^2x.cos^2x=1\)

Vậy...

3,\(C=\dfrac{1}{2}\left[cos\left(-\dfrac{7\pi}{12}\right)+cos\left(2x-\dfrac{\pi}{12}\right)\right]+\dfrac{1}{2}\left[cos\left(-\dfrac{7\pi}{12}\right)+cos\left(2x+\dfrac{11\pi}{12}\right)\right]\)

\(=cos\left(-\dfrac{7\pi}{12}\right)+\dfrac{1}{2}\left[cos\left(2x-\dfrac{\pi}{12}\right)+cos\left(2x+\dfrac{11\pi}{12}\right)\right]\)\(=\dfrac{-\sqrt{6}+\sqrt{2}}{4}+\dfrac{1}{2}\left[cos\left(2x-\dfrac{\pi}{12}\right)+cos\left(2x-\dfrac{\pi}{12}+\pi\right)\right]\)

\(=\dfrac{-\sqrt{6}+\sqrt{2}}{4}+\dfrac{1}{2}\left[cos\left(2x-\dfrac{\pi}{12}\right)-cos\left(2x-\dfrac{\pi}{12}\right)\right]\)\(=\dfrac{-\sqrt{6}+\sqrt{2}}{4}\)

Vậy...

4, \(D=cos^2x+\left(-\dfrac{1}{2}cosx-\dfrac{\sqrt{3}}{2}sinx\right)^2+\left(-\dfrac{1}{2}.cosx+\dfrac{\sqrt{3}}{2}.sinx\right)^2\)

\(=cos^2x+\dfrac{1}{4}cos^2x+\dfrac{\sqrt{3}}{4}cosx.sinx+\dfrac{3}{4}sin^2x+\dfrac{1}{4}cos^2x-\dfrac{\sqrt{3}}{4}cosx.sinx+\dfrac{3}{4}sin^2x\)

\(=\dfrac{3}{2}\left(cos^2x+sin^2x\right)=\dfrac{3}{2}\)

Vậy...

5, Xem lại đề

6,\(F=-cosx+cosx-tan\left(\dfrac{\pi}{2}+x\right).cot\left(\pi+\dfrac{\pi}{2}-x\right)\)

\(=tan\left(\pi-\dfrac{\pi}{2}-x\right).cot\left(\dfrac{\pi}{2}-x\right)\)\(=tan\left(\dfrac{\pi}{2}-x\right).cot\left(\dfrac{\pi}{2}-x\right)\)\(=cotx.tanx=1\)

Vậy...

d/

ĐKXĐ: ...

\(\Leftrightarrow cos^2x+\frac{1}{cos^2x}+2=2\left(cosx+\frac{1}{cosx}\right)\)

\(\Leftrightarrow\left(cosx+\frac{1}{cosx}\right)^2=2\left(cox+\frac{1}{cosx}\right)\)

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

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

\(\Rightarrow cosx=1\)

\(\Rightarrow x=k2\pi\)

c/

\(\Leftrightarrow cos\frac{6x}{5}+2=3cos\frac{4x}{5}\)

Đặt \(\frac{2x}{5}=a\)

\(\Rightarrow cos3a+2=3cos2a\)

\(\Leftrightarrow4cos^3a-3cosa+2=6cos^2a-3\)

\(\Leftrightarrow4cos^3a-6cos^2a-3cosa+5=0\)

\(\Leftrightarrow\left(cosa-1\right)\left(4cos^2a-2cosa-5\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}cosa=1\\cosa=\frac{1+\sqrt{21}}{4}>1\left(l\right)\\cosa=\frac{1-\sqrt{21}}{4}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}cos\left(\frac{2x}{5}\right)=1\\cos\left(\frac{2x}{5}\right)=\frac{1-\sqrt{21}}{4}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}\frac{2x}{5}=k2\pi\\\frac{2x}{5}=\pm arccos\left(\frac{1-\sqrt{21}}{4}\right)+k2\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=k5\pi\\x=\pm\frac{5}{2}arccos\left(\frac{1-\sqrt{21}}{4}\right)+k5\pi\end{matrix}\right.\)