  We describe the defining ideal of the rth secant variety of P^2 x P^n
embedded by O(1,2), for arbitrary n and r at most 5. We also present the Schur
module decomposition of the space of generators of each such ideal. Our main
results are based on a more general construction for producing explicit matrix
equations that vanish on secant varieties of products of projective spaces.
This extends previous work of Strassen and Ottaviani.
