In the given figure, we can see, LM || CB,
By using basic proportionality theorem, we get,
$\dfrac{AM}{AB} = \dfrac{AL}{AC}$……………………..(i)
Similarly, given, LN || CD and using basic proportionality theorem,
∴$\dfrac{AN}{AD} = \dfrac{AL}{AC}$……………………………(ii)
From equation (i) and (ii), we get,
$\dfrac{AM}{AB} = \dfrac{AN}{AD}$
Hence, proved.