Giải giúp mình với ạ, cần gấp!!!

$\begin{array}{l}
C = \dfrac{{\sin {{10}^o}\cos {{20}^o} + \sin {{20}^o}\cos {{10}^o}}}{{\cos {{17}^o}\cos {{13}^o} - \sin {{17}^o}\sin {{13}^o}}}\\
C = \dfrac{{\sin \left( {{{10}^o} + {{20}^o}} \right)}}{{\cos \left( {{{17}^o} + {{13}^o}} \right)}} = \dfrac{{\sin {{30}^o}}}{{\cos {{30}^o}}} = \tan {30^o} = \dfrac{{\sqrt 3 }}{3}\\
E = {\sin ^2}{20^o} + {\sin ^2}{100^o} + {\sin ^2}{140^o}\\
E = {\sin ^2}{20^o} + {\sin ^2}{40^o} + {\sin ^2}{80^o}\\
E = \dfrac{{1 - \cos {{40}^o}}}{2} + \dfrac{{1 - \cos {{80}^o}}}{2} + \dfrac{{1 - \cos {{160}^o}}}{2}\\
 = \dfrac{3}{2} - \left( {\dfrac{{\cos {{40}^o} + \cos {{80}^o} + \cos {{160}^o}}}{2}} \right)\\
 = \dfrac{3}{2} - \left( {\dfrac{{2\cos {{60}^o}\cos {{20}^o} - \cos {{20}^o}}}{2}} \right)\\
 = \dfrac{3}{2} - \left( {\dfrac{{\cos {{20}^o}\left( {2\cos {{60}^o} - 1} \right)}}{2}} \right) = \dfrac{3}{2}
\end{array}$