Equilateral triangle means each interior angle has measure 60°
We need the resultant which is our dipole
[tex]\\ \rm\Rrightarrow R=\sqrt{A^2+B^2+2ABcos\theta}[/tex]
[tex]\\ \rm\Rrightarrow R=\sqrt{(q\ell)²+(q\ell)²+2(q\ell)(q\ell)cos60°}[/tex]
[tex]\\ \rm\Rrightarrow R=\sqrt{(q\ell)^2+(q\ell)^2+(q\ell)^2}[/tex]
[tex]\\ \rm\Rrightarrow R=\sqrt{3(q\ell)^2}[/tex]
[tex]\\ \rm\Rrightarrow R=\sqrt{3}{q\ell}[/tex]
