a) Vì $20^{\circ}$ + $160^{\circ}$ = $180^{\circ}$
nên sin$20^{\circ}$ = sin($180^{\circ}$ - $160^{\circ}$) = sin$160^{\circ}$ (đpcm)
b) Vì $50^{\circ}$ + $130^{\circ}$ = $180^{\circ}$
nên cos$50^{\circ}$ = cos($180^{\circ}$ - $130^{\circ}$) = - cos$130^{\circ}$ (đpcm)