a: cosx=3
mà -1<=cosx<=1
nên \(x\in\varnothing\)
b: \(cosx=\dfrac{\sqrt{3}}{2}\)
=>\(cosx=cos\left(\dfrac{pi}{6}\right)\)
=>x=pi/6+k2pi hoặc x=-pi/6+k2pi
c: \(\Leftrightarrow\left[{}\begin{matrix}x+\dfrac{pi}{12}=\dfrac{3}{12}pi+k2pi\\x+\dfrac{pi}{12}=-\dfrac{3}{12}pi+k2pi\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{2}{12}pi+k2pi=\dfrac{1}{6}pi+k2pi\\x=-\dfrac{4}{12}pi+k2pi=-\dfrac{1}{3}pi+k2pi\end{matrix}\right.\)
d: =>x-20 độ=70 độ+k*360 hoặc x-20=-70+k*360
=>x=90+k*360 hoặc x=-50+k*360