sin 67° . cos 23° + cos 67° . sin 23° is equal to :

1. -1

2. 2 sin 67°

3. 2 cos 23°

4. 1

Show that :

tan 10° tan 15° tan 75° tan 80° = 1

Show that :

sin 42° sec 48° + cos 42° cosec 48° = 2