Đáp án:
Giải thích các bước giải:
1.A=$2\cos \frac{160^{\circ}+20^{\circ}}{2}.\cos \frac{160^{\circ}-20^{\circ}}{2}+\sin 30^{\circ}=-2\cos 90^{\circ}.\cos 70^{\circ}+\sin 30^{\circ}=\sin 30^{\circ}=\frac{1}{2}$
2.B=$(-\sin 40^{\circ}+\sin 40^{\circ})+(\sin ^{2}40^{\circ}+\cos ^{2}40^{\circ})=0+1=1$
3.C=$\frac{1+\cos 100^{\circ}}{2}+\frac{1+\cos 80^{\circ}}{2}-\frac{\sin 10^{\circ}.\sin 80^{\circ}}{\cos 10^{\circ}.\cos 80^{\circ}}$
=$\frac{1+\cos 100^{\circ}}{2}+\frac{1+\cos 80^{\circ}}{2}-\frac{\frac{\cos 70^{\circ}}{2}}{\frac{\cos 70^{\circ}}{2}}$
=$1+\frac{1}{2}.(\cos 100^{\circ}+\cos 80^{\circ})-1$
=$\frac{1}{2}.2.\cos 90^{\circ}.\cos 10^{\circ}=0$
