Let △ABC and △ABD be on the same base AB and between the same parallels AB and CD.Then the relation between areas of △ABC and △ABD will be:

Area $(\triangle ABC)$ = Area $(\triangle ABD)$

We know that ,

Area of triangle = \(\frac{1}{2}\) × Base × Height

△ABC and △ABD , both triangles are on same base i.e. AB

And point C and D both are on also on same line ( perpendicular distance between both the line is same )

⇒ Height is also same.

So, Area of  △ABC = Area of △ABD