Rtđ=R+Rb
PR=[tex]I^{2}.R=(\frac{U}{Rtd})^{2}.R[/tex]=[tex]\frac{U^{2}.R}{(R+Rb)^{2}}[/tex]
Ta có:
[tex]\frac{U^{2}.R}{(R+Rb)^{2}}[/tex][tex]\leq \frac{U^{2}.R}{4.R.Rb}[/tex]
=>[tex]PRmax=\frac{U^{2}}{4Rb}[/tex]
Dấu bằng xảy ra khi R=Rb=12[tex]\Omega[/tex]