I’m one who really hates memorizing formulas, be in as a student or tutor. So I shall share how I remember the three very useful trigonometry identities. I’ll start with the one which you all definitely can identify with.
Now how do we get the other 2?
Simply divide the above equation by and respectively.
Lets divide by !
Voila! we have
You can try to do the same with ! Let me know if you have problems!