Let's limit our argument for positive numbers:
a+-wa * b+-wb = [(a-wa)*(b-wb), (a+wa)*(b+wb)]
[a*b - a*wb - b*wb + wa*wb, a*b + a*wb + b*wa + wa*wb]
if we assume wa*wb is insignificantly small, we get:
a*b+-(a*wb+b*wa)

tolerances are the following:
ta = wa/a
tb = wb/b
tab = (a*wb+b*wa)/(a*b) = (a*b*tb + a*b*ta)/(a*b) = ta+tb