2а/с................................................
<span>cos72cos42+sin72sin42=сos(72-42)=cos30=</span>√3/2
1
π(2x-1)/3=-π/3+2πn U π(2x-1)/3=π/3+2πn
2x-1=-1+6n U 2x-1=1+6n
2x=6n U 2x=2+6n
x=3n U x=1+3n,n∈z
3
4sin76/sin(360-76)=4sin76/(-sin76)=-4
4
-24tg(90-20)*tg(180-20)=-24*ctg20*(-tg20)=-24*(-1)=24