1/R=1/R1+1/R2
1/R1=1/R-1/R2
1/R1=(R2-R)/RR2
R1=RR2/(R2-R)第3步什么意思？1/R1=1/R-1/R2通分即得：1/R1=R2/RR2-R/RR2==(R2-R)/RR2