Question

Giúp em câu 39 ạ

Answer 1

37.

\(C^3_{13}-C^3_9-C^3_8-C^3_9+C^3_5+C^3_4+C^3_4=80\)

Answer 2

39. 

\(cos4x+2sin6x=2\sqrt{3}sin3x.cosx+cos2x\)

\(\Leftrightarrow cos4x-cos2x+4sin3x.cos3x-2\sqrt{3}sin3x.cosx=0\)

\(\Leftrightarrow-2sin3x.sinx+2sin3x.\left(2cos3x-\sqrt{3}cosx\right)=0\)

\(\Leftrightarrow2sin3x.\left(2cos3x-\sqrt{3}cosx-sinx\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sin3x=0\\sinx+\sqrt{3}cosx=2cos3x\end{matrix}\right.\)