Toán Lớp 11: Chứng minh đẳng thức sau :
sin5x – 2sinx . (cos4x + cos2x) =sinx
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Toán Lớp 11: Chứng minh đẳng thức sau :
sin5x – 2sinx . (cos4x + cos2x) =sinx
Comments ( 2 )
\sin 5x – 2\sin x\left( {\cos 4x + \cos 2x} \right)\\
= \sin 5x – 2\sin x\cos 4x – 2\sin x\cos 2x\\
= \sin 5x – \left( {\sin 5x – \sin 3x} \right) – \left( {\sin 3x – \sin x} \right)\\
= \sin 5x – \sin 5x + \sin 3x – \sin 3x + \sin x\\
= \sin x
\end{array}$