Proving that Selection Operator on Cartesian Product of Two Relations is Equivalent to Theta-Join on Those Same Realtions

Question

As the title suggests, I am trying to prove that $\sigma_\theta(R1\times R2) =R1 \bowtie_\theta R2$.
However, as far as I know, the very definition of $\theta$-join is the LHS equation from above, so I am quite confused as to
how I should generate a (semi) formal prove here. Any suggestions will be greatly appreciated.

Answer 1

Prove that any tuple that appear in the LHS will appear in the RHS and viceversa.