§ 25: 25.21

25.21 Доведіть, що tg75° – ctg75° = 23.

sin75°cos75° – cos75°sin75° = sin275° - cos275°cos75° sin75° = -2cos150°2cos75°sin75° = –2cos150°sin150° = –2tg150° = –2 • tg(180° – 30°) = 2tg30° = 23.