Question

Tính số đó góc x biết

\(cos^2x-2sin^2x=\dfrac{1}{4}\)

Answer 1

ta có : \(cos^2x-2sin^2x=\dfrac{1}{4}\Leftrightarrow1-3sin^2x=\dfrac{1}{4}\)

\(\Leftrightarrow sin^2x=\dfrac{1}{4}\Leftrightarrow sinx=\pm\dfrac{1}{2}\) \(\Rightarrow x=30^o\overset{.}{,}x=330^o\)

ta có : \(cos^2x-2sin^2x=\dfrac{1}{4}\Leftrightarrow3cos^2x-2=\dfrac{1}{4}\)

\(\Leftrightarrow cos^2x=\dfrac{3}{4}\Leftrightarrow cosx=\pm\dfrac{\sqrt{3}}{2}\) \(\Rightarrow x=30^o\overset{.}{,}x=150^o\)

vậy\(x=30^o\overset{.}{,}x=330^o\overset{.}{,}x=150\)