R0 nt (R1 //R2)(gọi điện trở R2 là x(ôm)
\(=>Pab=Iab^2.Rab=\left(\dfrac{U}{Rtd}\right)^2\left(\dfrac{12x}{12+x}\right)=\left(\dfrac{12^{ }}{Ro+\dfrac{12x}{12+x}}\right)^2\left(\dfrac{12x}{12+x}\right)\)
\(=>Pab=\dfrac{12^2.12x}{\left(3+\dfrac{12x}{12+x}\right)^2\left(12+x\right)}=\dfrac{1728x}{\left(\dfrac{15x+36}{12+x}\right)^2\left(12+x\right)}=\dfrac{1728x}{\dfrac{\left(15x+36\right)^2}{12+x}}\)
\(=\dfrac{1728x\left(12+x\right)}{\left(15x+36\right)^2}\)\(\le12\) dấu"=" xảy ra<=>x=R2=4(ôm)
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