1.5UR=UR+12UR=24V 设在RL两端的电压为U,电流为U/RL主线电流为4+U/RL=(1.5UR-U)/1=36-U所以U/RL+U=32U=32RL/(1+RL)P=U^2/RL=32^2RL/(1+RL)^2=32^2/(2+RL+1/RL)RL+1/RL>=2 当且仅当RL=1时有最小值2,所以1/(2+RL+1/RL)的最大值是1/4所以Pmax=8*32=256w,此时RL=1欧姆