Antwort
R= R1 * R2 / (R1 + R2) | * (R1 + R2)
R* (R1 + R2) = R1 * R2 | Klammer auflösen
R* R1 + R * R2 = R1 * R2 l - (R * R1)
R* R2 = R1*Rz - R * R1 | R1 herausheben
R* R2 = R1* (R2 - R) | / (R2 - R)
R1= R * R2 / R2 - R