R1,R2=var ('R1,R2')
R(R1,R2)=R1*R2/(R1+R2)
dR1,dR2=var ('dR1,dR2')
totdiff=diff(R,R1)*dR1+diff(R,R2)*dR2
show(totdiff)
dR1=2.5
dR2=10
partR1=diff(R,R1)
partR2=diff(R,R2)
dR=partR1(100,400)*dR1+partR2(100,400)*dR2
show(dR)
rel=dR/R(100,400)*100
show(rel)