考研电路真题,麻烦各位帮我看看第七题怎么做,

设2欧电阻的电流为I3,方向向右,则：U2=nU1i1=I2nI3=(10-U1)/1-I1 I2=I3-U2/RI3=(U1-U2)/2联立解得：U1=20n/(-n^2+2n+2)U2=nU1=20n^2/(-n^2+2n+2)R上的功率为：P2=U2^2/RP2最大,即P2的导数为0P2'=20{[n^2/(-n^2+2n+2)]^2}'=020{2n/(-n^2+2n+2)]^2-n^2/(-n^2+2n+2)]^4*2(-n^2+2n+2)*(-2n+2)=0得：n^2=2n=1。414U2=nU1=1。414*最大功率为P2=P2=U2^2/R=(nU1)^2=[20n/(-n^2+2n+2)]^2=100W