I'm trying to find the answer to 1/r1 + 1/r2= 1/R on a calculator but it says it has no solution.

Answer 1

using common denominator ... [r2 / (r1 * r2)] + [r1 / (r1 * r2)] = 1 / R

combining terms ... (r1 + r2) / (r1 * r2) = 1 / R

inverting ... (r1 * r2) / (r1 + r2) = R