a)

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

\(=(\sin ^2x+\cos ^2x)+2\sin x\cos x=1+2\sin x\cos x\)

b)

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

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

\(=1-2\sin ^2x\cos ^2x\)

c)

\(\tan ^2x-\sin ^2x=(\frac{\sin x}{\cos x})^2-\sin ^2x\)

\(=\sin ^2x\left(\frac{1}{\cos ^2x}-1\right)=\sin ^2x. \frac{1-\cos ^2x}{\cos ^2x}=\sin ^2x.\frac{\sin ^2x}{\cos ^2x}\)

\(=\sin ^2x\left(\frac{\sin x}{\cos x}\right)^2=\sin ^2x\tan ^2x\)

d)

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

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

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

\(=(\sin ^4x+\cos ^4x)-\sin ^2x\cos ^2x=1-2\sin ^2x\cos ^2x-\sin ^2x\cos ^2x\)

\(=1-3\sin ^2x\cos ^2x\) (theo kq phần b)

e)

\(\sin x\cos x(1+\tan x)(1+\cot x)=\sin x\cos x(1+\frac{\sin x}{\cos x})(1+\frac{\cos x}{\sin x})\)

\(=\sin x\cos x.\frac{\cos x+\sin x}{\cos x}.\frac{\sin x+\cos x}{\sin x}\)

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

\(=1+2\sin x\cos x\)

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P/s: Nói chung cứ bám vào công thức \(\sin ^2x+\cos ^2x=1\)

sin x + cos x = 1 + sin x.cos x

⇔ sin x.cos x – sin x – cos x + 1 = 0

⇔ (sinx. cosx –sinx)- (cosx -1 ) =0

⇔ sinx. (cosx – 1) – (cosx -1) = 0

⇔ (sin x – 1)(cos x – 1) = 0

Vậy phương trình có tập nghiệm 

\(\Leftrightarrow2cosx-sinx-4sin^2x.cosx+2sin^3x=sin^3x+cos^3x\)

\(\Leftrightarrow sin^3x-cos^3x-4sin^2x.cosx+2cosx-sinx=0\)

- Với \(\left\{{}\begin{matrix}cosx=0\\sinx=1\end{matrix}\right.\) \(\Leftrightarrow x=\frac{\pi}{2}+k2\pi\) là nghiệm của pt

- Với \(cosx\ne0\) chia 2 vế cho \(cos^3x\)

\(tan^3x-1-4tan^2x+2\left(1+tan^2x\right)-tanx\left(1+tan^2x\right)=0\)

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

\(\Rightarrow\left[{}\begin{matrix}tanx=1\\tanx=-\frac{3}{2}\end{matrix}\right.\)

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

cot x=2>0

=>sin x và cosx cùng dấu

=>sinx*cosx>0

\(1+cot^2x=\dfrac{1}{sin^2x}=1+4=5\)

=>sin^2x=1/5

=>cos^2x=4/5

\(B=\dfrac{1}{5}-2\cdot sinx\cdot cosx-\dfrac{1}{5}\cdot\dfrac{4}{5}+\dfrac{1}{5}-3\)

\(=\dfrac{2}{5}-\dfrac{4}{25}-3-2\cdot\dfrac{1}{\sqrt{5}}\cdot\dfrac{2}{\sqrt{5}}\)

\(=\dfrac{10}{25}-\dfrac{4}{25}-\dfrac{75}{25}-2\cdot\dfrac{2}{5}=\dfrac{-69}{25}-\dfrac{4}{5}=\dfrac{-89}{25}\)

cotx=2

=>cosx=2*sin x

\(1+cot^2x=\dfrac{1}{sin^2x}\)

=>\(\dfrac{1}{sin^2x}=1+4=5\)

=>\(sin^2x=\dfrac{1}{5}\)

\(B=\dfrac{sin^2x-2\cdot sinx\cdot2\cdot sinx-1}{5\cdot4sin^2x+sin^2x-3}=\dfrac{-3sin^2x-1}{21sin^2x-3}\)

\(=\dfrac{-\dfrac{3}{5}-1}{\dfrac{21}{5}-3}=-\dfrac{8}{5}:\dfrac{6}{5}=-\dfrac{4}{3}\)

\(cotx=2\Rightarrow tanx=\dfrac{1}{2}\)

\(B=\dfrac{sin^2x-2sinx.cosx-1}{5cos^2x+sin^2x-3}\)

\(\Leftrightarrow B=\dfrac{tan^2x-2tanx-\dfrac{1}{cos^2x}}{5+tan^2x-\dfrac{3}{cos^2x}}\)

\(\Leftrightarrow B=\dfrac{tan^2x-2tanx-1-tan^2x}{5+tan^2x-3-3tan^2x}\)

\(\Leftrightarrow B=\dfrac{-2tanx-1}{2-2tan^2x}\)

\(\Leftrightarrow B=\dfrac{-2.\dfrac{1}{2}-1}{2-2.\dfrac{1}{4}}=\dfrac{-2}{\dfrac{3}{2}}=-\dfrac{4}{3}\)

Đề như vậy hả bạn: \(\frac{3cosx+4sinx+6}{3cosx+4sinx+1}=2\)

Câu 2:

\(A=2\cdot\dfrac{1}{2}+3\cdot\dfrac{1}{2}+1=1+1+1=3\)

Bài 3:

\(cos^2a=1-\left(\dfrac{12}{13}\right)^2=\dfrac{25}{169}\)

mà cosa>0

nên cosa=5/13

=>tan a=12/5; cot a=5/12

Câu 4: \(sin^2a=1-\dfrac{1}{4}=\dfrac{3}{4}\)

mà sina <0

nên sin a=-căn 3/2

=>tan a=-căn 3

\(A=-\dfrac{\sqrt{3}}{2}+\dfrac{1}{2}\cdot\left(-\sqrt{3}\right)=-\sqrt{3}\)