Question

cos(70°+x)*cos(x-20°)=1/2

Answer 1

$cos(70а+x)cos(x-20а)=0.5\ \ cos(70а+x)cos(70а+x-90а)=0.5\ \ cos(70а+x)sin(70а+x)=0.5|cdot 2\ \ 2cos(70а+x)sin(70а+x)=1\ \ sin(140а+2x)=1\ \ 140а+2x=90а+360аk,k in Z\ \ 2x=-50а+360аk,k in Z\ \ x=-25а+180аk,k in Z$