Question

sin $pi /5$ * cos$pi /20$ + cos $pi /5$ * sin$pi /20$

Answer 1

$sin frac{pi}{5} cos frac{pi}{20} + cos frac{pi}{5} sin frac{pi}{20}= sin( frac{pi}{5}+frac{pi}{20})= \ =sin( frac{4pi}{20}+frac{pi}{20})=sin frac{5pi}{20}=sin frac{pi}{4}= frac{ sqrt{2} }{2}$