I think Creo is right.
I'm not sure where the formula came from but ...
L = l_base - ((l_base-l_top)/delta_z) * Z
W = w_base - ((w_base-w_top)/delta_z) * Z
A = L * W =
l_base*w_base
- (l_base*((w_base-w_top)/delta_z) + w_base*((l_base-l_top)/delta_z)) * Z
+ ((l_base-l_top)/delta_z)*((w_base-w_top)/delta_z)*Z*Z
Integrating:
V =
l_base*w_base*Z
- (l_base*((w_base-w_top)/delta_z) + w_base*((l_base-l_top)/delta_z)) * Z*Z/2
+ ((l_base-l_top)/delta_z)*((w_base-w_top)/delta_z)*Z*Z*Z/3
Plugging it all in I get 11.33...