Cho doan mach go $R$,$L$,$C$ mac noi tiep,co $RC^{2}<2L$.Dat vao 2 dau doan mach mot dien ap khong doi $U=U\sqrt{2}cos(2\pi ft)$.Thay doi f ta thay khi f=f1 thi $Uc$max va cong suat luc nay bang $\frac{3}{4}$ cong suat cuc dai.Khi f=f2=f1+100Hz thi $Ul$max.Tim tan so luc cong suat cuc dai ?