Что это за формула? r=r1(r2+r3+r4)/r1+r2+r3

докажите тождество 1-cos2 t/ 1- sin2 t + tg t * ctg t = 1/cos2 t

do you know how to use these words?

this
these
here
come
that
those
there
go

I'd like to come / go away for a holiday again soon.
You must come/ go and see us again one of these days.
Let's all come/ go and see Harry this weekend.
I've found something very strange. Come / go and have a look.
I'm afraid Mrs. Barnes is busy just now. Could you come / go back tomorow morning.
Do you know Africa? - No , I' ve never been here/ theere
-This / that is my first visit!
I'll never forget this/ that morning , 20 years ago when I first saw Mrs. Newton .
Jane, I'd like you to meet Peter. Peter , this / that is my friend Jane
Who were these/ those people you were with last night.