8: \(B=-\dfrac{1}{2}\cdot\left[cos72-cos60\right]\cdot\left(-\dfrac{1}{2}\right)\cdot\left[cos120-cos36\right]\)
\(=\dfrac{1}{4}\cdot\left[cos72-\dfrac{1}{2}\right]\left[-\dfrac{1}{2}-cos36\right]\)
\(=\dfrac{1}{4}\cdot\left[-\dfrac{1}{2}\cdot cos72-cos72\cdot cos36+\dfrac{1}{4}+\dfrac{1}{2}\cdot cos36\right]\)
\(=\dfrac{1}{4}\left[\dfrac{1}{4}-\dfrac{1}{2}\cdot\left(2cos^236^0-1\right)-\left(2cos^236^0-1\right)\cdot cos36^0+\dfrac{1}{2}\cdot cos36\right]\)
=1/4*1/4=1/16
10: \(\Leftrightarrow B_{10}\cdot sin20=sin20\cdot cos20\cdot cos40\cdot cos60\cdot cos80\)
\(=\dfrac{1}{2}\cdot sin40\cdot cos40\cdot cos80\cdot\dfrac{1}{2}\)
\(=\dfrac{1}{2}\cdot\dfrac{1}{2}\cdot2\cdot sin40\cdot cos40\cdot cos80\cdot\dfrac{1}{2}\)
\(=\dfrac{1}{8}\cdot sin80\cdot cos80\)
=1/16*sin160
=>B10=1/16
giúp em với ạ 
