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a/ \(cos\left(x+15^0\right)=1\Leftrightarrow x+15^0=k360^0\Rightarrow x=-15^0+k360^0\)

b/ \(cos\left(3x+\frac{\pi}{3}\right)=\frac{\sqrt{2}}{2}\Rightarrow\left[{}\begin{matrix}3x+\frac{\pi}{3}=\frac{\pi}{4}+k2\pi\\3x+\frac{\pi}{3}=-\frac{\pi}{4}+k2\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=-\frac{\pi}{36}+\frac{k2\pi}{3}\\x=-\frac{7\pi}{36}+\frac{k2\pi}{3}\end{matrix}\right.\)

c/ \(cos\left(4x-\frac{\pi}{4}\right)=-\frac{\sqrt{2}}{3}\Rightarrow cos\left(4x-\frac{\pi}{4}\right)=cosa\)

\(\Rightarrow\left[{}\begin{matrix}4x-\frac{\pi}{4}=a+k2\pi\\4x-\frac{\pi}{4}=-a+k2\pi\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{16}+\frac{a}{4}+\frac{k\pi}{2}\\x=\frac{\pi}{16}-\frac{a}{4}+\frac{k\pi}{2}\end{matrix}\right.\)

d/ \(cos4x=cos\left(x+\frac{\pi}{3}\right)\Rightarrow\left[{}\begin{matrix}x+\frac{\pi}{3}=4x+k2\pi\\x+\frac{\pi}{3}=-4x+k2\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{9}+\frac{k2\pi}{3}\\x=-\frac{\pi}{15}+\frac{k2\pi}{5}\end{matrix}\right.\)

e/ \(cos5x=-cos3x=cos\left(\pi-3x\right)\Rightarrow\left[{}\begin{matrix}5x=\pi-3x+k2\pi\\5x=3x-\pi+k2\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=\frac{\pi}{8}+\frac{k\pi}{4}\\x=-\frac{\pi}{2}+k\pi\end{matrix}\right.\)

đọc lại lý thuyết rồi làm 

a, (sinx + cosx)(1 - sinx . cosx) = (cosx - sinx)(cosx + sinx)

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

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

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

⇔ \(\left[{}\begin{matrix}sin\left(x+\dfrac{\pi}{4}\right)=0\\cosx=1\\sinx=-1\end{matrix}\right.\)

b, (sinx + cosx)(1 - sinx . cosx) = 2sin2x + sinx + cosx

⇔ (sinx + cosx)(1 - sinx.cosx - 1) = 2sin2x

⇔ (sinx + cosx).(- sinx . cosx) = 2sin2x

⇔ 4sin2x + (sinx + cosx) . sin2x = 0

⇔ \(\left[{}\begin{matrix}sin2x=0\\\sqrt{2}sin\left(x+\dfrac{\pi}{4}\right)+4=0\end{matrix}\right.\)

⇔ sin2x = 0

c, 2cos³x = sin3x

⇔ 2cos³x = 3sinx - 4sin³x

⇔ 4sin³x + 2cos³x - 3sinx(sin²x + cos²x) = 0

⇔ sin³x + 2cos³x - 3sinx.cos²x = 0

Xét cosx = 0 : thay vào phương trình ta được sinx = 0. Không có cung x nào có cả cos và sin = 0 nên cosx = 0 không thỏa mãn phương trình

Xét cosx ≠ 0 chia cả 2 vế cho cos³x ta được : 

tan³x + 2 - 3tanx = 0

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

d, cos²x - \(\sqrt{3}sin2x\) = 1 + sin²x

⇔ cos²x - sin²x - \(\sqrt{3}sin2x\) = 1

⇔ cos2x - \(\sqrt{3}sin2x\) = 1

⇔ \(2cos\left(2x+\dfrac{\pi}{3}\right)=1\)

⇔ \(cos\left(2x+\dfrac{\pi}{3}\right)=\dfrac{1}{2}=cos\dfrac{\pi}{3}\)

e, cos³x + sin³x = 2cos⁵x + 2sin⁵x

⇔ cos³x (1 - 2cos²x) + sin³x (1 - 2sin²x) = 0

⇔ cos³x . (- cos2x) + sin³x . cos2x = 0

⇔ \(\left[{}\begin{matrix}sin^3x=cos^3x\\cos2x=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}sinx=cosx\\cos2x=0\end{matrix}\right.\)

⇔ \(\left[{}\begin{matrix}sin\left(x-\dfrac{\pi}{4}\right)=0\\cos2x=0\end{matrix}\right.\)

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

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

\(\Leftrightarrow\left(sinx+cosx\right)\left(1-sinx.cosx\right)-2sin^2x.cos^2x\left(sinx+cosx\right)=0\)

\(\Leftrightarrow\left(sinx+cosx\right)\left(1-\frac{1}{2}sin2x-\frac{1}{2}sin^22x\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sinx+cos=0\\1-\frac{1}{2}sin2x-\frac{1}{2}sin^22x=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}sin\left(x+\frac{\pi}{4}\right)=0\\sin2x=1\\sin2x=-2\left(l\right)\end{matrix}\right.\)

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

1: \(\sin\left(x+\frac{\pi}{4}\right)=\frac23\)

=>\(\left[\begin{array}{l}x+\frac{\pi}{4}=\arcsin\left(\frac23\right)+k2\pi\\ x+\frac{\pi}{4}=\pi-\arcsin\left(\frac23\right)+k2\pi\end{array}\right.\Rightarrow\left[\begin{array}{l}x=\arcsin\left(\frac23\right)-\frac{\pi}{4}+k2\pi\\ x=\frac34\pi-\arcsin\left(\frac23\right)+k2\pi\end{array}\right.\)

2: \(cos2x-5\cdot\sin x-3=0\)

=>\(1-2\cdot\sin^2x-5\cdot\sin x-3=0\)

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

=>\(2\cdot\sin^2x+5\cdot\sin x+2=0\)

=>(sin x+2)(2 sin x+1)=0

=>2 sin x+1=0

=>sin x=-1/2

=>\(\left[\begin{array}{l}x=-\frac{\pi}{6}+k2\pi\\ x=\pi+\frac{\pi}{6}+k2\pi=\frac76\pi+k2\pi\end{array}\right.\)

3: \(cos3x=\sin2x\)

=>\(cos3x=cos\left(\frac{\pi}{2}-2x\right)\)

=>\(\left[\begin{array}{l}3x=\frac{\pi}{2}-2x+k2\pi\\ 3x=2x-\frac{\pi}{2}+k2\pi\end{array}\right.\Rightarrow\left[\begin{array}{l}5x=\frac{\pi}{2}+k2\pi\\ x=-\frac{\pi}{2}+k2\pi\end{array}\right.\)

=>\(\left[\begin{array}{l}x=\frac{\pi}{10}+\frac{k2\pi}{5}\\ x=-\frac{\pi}{2}+k2\pi\end{array}\right.\)

Chọn gốc tọa độ trùng với điểm A(0;0;0);

Tia AB nằm trên trục Ox.

Tia AD nằm trên trục Oy.

Tia Az (chứa SA) nằm trên trục Oz.

SA=2

=>S(0;0;2)

AB=1

=>B(1;0;0)

AD=3

=>D(0;3;0)

ABCD là hình bình hành

=>BC=AD=3

=>C(1;3;0)

Tọa độ trung điểm M của BC là:

\(\begin{cases}x=\frac{1+1}{2}=\frac22=1\\ y=\frac{0+3}{2}=\frac32=1,5\\ z=\frac{0+0}{2}=0\end{cases}\)

=>M(1;1,5;0)

S(0;0;2); A(0;0;0); M(1;1,5;0)

\(\overrightarrow{SA}=\left(0-0;0-0;0-2\right)=\left(0;0;-2\right)\)

\(\overrightarrow{AM}=\left(1-0;1,5-0;0-0\right)=\left(1;1,5;0\right)\)

\(\overrightarrow{SA}\cdot\overrightarrow{AM}=0\cdot1+0\cdot1,5+2\cdot0=0\)

=>\(cos\left(SA;AM\right)=0\)

b: S(0;0;2); C(1;3;0); A(0;0;0); D(0;3;0)

\(\overrightarrow{SC}=\left(1-0;3-0;0-2\right)=\left(1;3;-2\right)\) ; \(\overrightarrow{AD}=\left(0-0;3-0;0-0\right)=\left(0;3;0\right)\)

\(SC=\sqrt{1^2+3^2+\left(-2\right)^2}=\sqrt{1+4+9}=\sqrt{14}\) ; \(AD=\sqrt{0^2+3^2+0^2}=3\)

\(cos\left(SC;AD\right)=\frac{\overrightarrow{SC}\cdot\overrightarrow{AD}}{SC\cdot AD}=\frac{1\cdot0+3\cdot3+\left(-2\right)\cdot0}{\sqrt{14}\cdot3}=\frac{9}{3\sqrt{14}}=\frac{3}{\sqrt{14}}\)

c: S(0;0;2); B(1;0;0); A(0;0;0); C(1;3;0)

\(\overrightarrow{SB}=\left(1-0;0-0;0-2\right)=\left(1;0;-2\right)\) ; \(\overrightarrow{AC}=\left(1-0;3-0;0-0\right)=\left(1;3;0\right)\)

\(SB=\sqrt{1^2+0^2+\left(-2\right)^2}=\sqrt5;AC=\sqrt{1^2+3^2+0^2}=\sqrt{1+9}=\sqrt{10}\)

cos(SB;AC)=\(\frac{\overrightarrow{SB}\cdot\overrightarrow{AC}}{\left|\overrightarrow{SB}\right|\cdot\left|\overrightarrow{AC}\right|}\)

\(\frac{1\cdot1+0\cdot3+\left(-2\right)\cdot0}{\sqrt5\cdot\sqrt{10}}=\frac{1}{\sqrt{50}}=\frac{1}{5\sqrt2}=\frac{\sqrt2}{10}\)