We can re-draw the fig.1.15(as shown above) where we add DO which will be height of $\triangle B C D$. Now, $\frac{ A (\triangle ABC )}{ A (\triangle BCD )}=\frac{ AP }{ DO }$ (PROPERTY: Areas of triangles with equal bases are proportional to their corresponding heights.) $\begin{aligned} \Rightarrow & \frac{A(\triangle A B C)}{A(\triangle B C D)}=\frac{A P}{D O} \\ \Rightarrow & \frac{A(\triangle A B C)}{A(\triangle B C D)}=\frac{1}{1} \end{aligned}$ ( $\because$ the distance between the two parallel lines is always equal $\Rightarrow A P=D O)$ $\Rightarrow \frac{ A (\triangle ABC )}{ A (\triangle BCD )}=1: 1$
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