Question

cos7xcos2x-sin7xsin2x=0

Answer 1

$cos7xcos2x-sin7xsin2x=0$
$cos(7x+2x)=0$
$cos9x=0$
$9x= frac{ pi }{2}+ pi k,$ k∈Z

$x= frac{ pi }{18}+ frac{ pi }{9} k,$ k∈Z