Câu 21:
\(1+tan^2\dfrac{x}{2}=\dfrac{1}{cos^2\dfrac{x}{2}}\)\(\Leftrightarrow1+\left(\sqrt{2}\right)^2=\dfrac{1}{cos^2\dfrac{x}{2}}\)
\(\Leftrightarrow cos^2\dfrac{x}{2}=\dfrac{1}{3}\)
Có \(cosx=2cos^2\dfrac{x}{2}-1=-\dfrac{1}{3}\)
Ý C
Câu 22:B
Câu 23:
\(A=4sinx.sin2x.sin3x=\left(2.sinx.sin2x\right).2sin3x\)
\(=\left(cosx-cos3x\right).2sin3x\)
\(=2sin3x.cosx-2sin3x.cos3x\)
\(=sin2x+sin4x-sin6x\) =>Ý D
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