Em vẽ hình theo cách gọi của nhé:
\[\begin{array}{l}
Dat\,\,\,AB = a;\,\,\,BC = x.\\
Goi\,\,\,cac\,\,chieu\,\,cao\,\,\,la\,\,AH\,\,\,va\,\,BK.\\
Khi\,\,do:\,\,\,AB = AC = x;\,\,\,BK = 12;\,\,\,AH = 15,6\\
\Rightarrow AK = \frac{1}{2}AC = \frac{a}{2}.\\
BH = \frac{1}{2}BC = \frac{x}{2}.\\
Ap\,\,dung\,\,\,dinh\,\,ly\,\,Pitago\,\,co:\\
A{B^2} = B{K^2} + K{A^2}\\
\Leftrightarrow {a^2} = {12^2} + \frac{{{a^2}}}{4}\\
\Leftrightarrow \frac{{3{a^2}}}{4} = 144 \Leftrightarrow {a^2} = 192.\\
B{H^2} = A{B^2} - A{H^2} = {a^2} - {12^2}\\
= 192 - 144 = 48.\\
\Rightarrow BH = \sqrt {48} = 4\sqrt 3 .
\end{array}\]
