lnPI=2RI*ln[US/(Rs+RI)]=2RI(lnUS-ln(RS+RI)]
对上式求导后
(1/pi)dpi/dri=2ln[US/(Rs+RI)]-2RI/(Rs+RI)
dpi/dri={2ln[US/(Rs+RI)]-2RI/(Rs+RI)}(Us/(Rs+Rl))^2*Rl
lnPI=2RI*ln[US/(Rs+RI)]=2RI(lnUS-ln(RS+RI)]
对上式求导后
(1/pi)dpi/dri=2ln[US/(Rs+RI)]-2RI/(Rs+RI)
dpi/dri={2ln[US/(Rs+RI)]-2RI/(Rs+RI)}(Us/(Rs+Rl))^2*Rl