clear all
sysuse auto

reg price c.weight##c.displacement
local b0 = _b[_cons]
local b1 = _b[displacement]
local b3 = _b[c.weight#c.displacement]

generate wd = weight * displacement
reg wd disp
local b4 = _b[_cons]
local b5 = _b[displacement]
predict wr, resid

reg price weight displacement wr
di `b0' + `b4'*`b3'
di `b1' + `b5'*`b3'