It is given that
\[~\left( \mathbf{4a}\text{ }+\text{ }\mathbf{5b} \right)\text{ }\left( \mathbf{4c}\text{ }\text{ }\mathbf{5d} \right)\text{ }=\text{ }\left( \mathbf{4a}\text{ }\text{ }\mathbf{5d} \right)\text{ }\left( \mathbf{4c}\text{ }+\text{ }\mathbf{5d} \right)\]
We can write it as
Therefore, it is proved that a, b, c, d are in proportion.