Make R1 the subject of this equation

R = R1xR2/R1+R2
and the numbers are small (not powers)

you have a mistake in your equation, but luckily I recognize it.

should be
R = R1xR2 / (R1+R2)
the difference is important.

And since I have done this before
R1 = R•R2 / (R–R2)

R = [(R1)*(R2)] / (R1 + R2)
Solve for R1 and get
R1 = R / [1 - (R/R2)]

I believe that if you use R1 = [(R)*(R2)] / (R - R2). Then R1 would always be negative since R is always less than R2.. Resistance values can not be negative.