Areas of triangles with the same bases along the same straight line are in the ratio of their bases.
Here in the figure the triangles △ABD, △BCD and △ACD have the same vertex D and bases along the same straight line AC, therefore they have the same height. In such cases the areas of the triangles are in the ratio of their bases.
⇒Area of triangle △ABDArea of triangle △BCD=ABBC
⇒Area of triangle △ABDArea of triangle △ACD=ABAC
⇒Area of triangle △BCDArea of triangle △ACD=BCAC