Please help me. I'm trying to find the answer to 1/r1 + 1/r2= 1/R for R. It's urgent. Sorry.

Answer 1

I guess you haven't bee trying too hard.
It has been done for you half a dozen times, and you have apparently not done any work so far.
Work it just like any other fractions

1/3 + 1/4 = (3+4)/(3*4)
Now take the reciprocal

Answer 2

This is getting strange.