goi x la chu so hang chuc.ĐK:1<= x <= 9 \(x\in N^{\cdot}\)

khi do: chu so hang don vi la: 2x

so ban dau la: 10x+2x= 12x

vi them so 1 vao giua 2 so nen ta co so moi la: 100x + 10 +2x= 102x +10

theo de ta co pt: 102x +10 -12x= 370

\(\Leftrightarrow90x=360\)

\(\Leftrightarrow x=4\left(TMĐK\right)\)

Vay so ban dau la: 12.4=48