Let $S_{N,N}$ be the Riemann sum for $\int_0^1\int_0^1 3 e^{x^3-y^3}\,dy\,dx$ using the regular partition and the lower left-hand vertex of each subrectangle as sample points.
Use a computer algebra system to calculate $S_{N,N}$ for $N=39, 64, 114$.

$S_{39,39}=$
$S_{64,64}=$
$S_{114,114}=$