Question

giải các pt

a) \(cos2x-cos40^o=0\)

b) \(cos3x-cos\left(x-80^o\right)=0\)

Answer 1

\(cos2x=cos40\)

\(\Rightarrow\left[{}\begin{matrix}2x=40^0+k360^0\\2x=-40^0+k360^0\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=20^0+k180^0\\x=-20^0+k180^0\end{matrix}\right.\)

\(cos3x=cos\left(x-80^0\right)\)

\(\Leftrightarrow\left[{}\begin{matrix}3x=x-80^0+k360^0\\3x=80^0-x+k360^0\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=-40^0+k180^0\\x=20^0+k90^0\end{matrix}\right.\)