"If a>0 then 1/a>0." (see ita-2-2-13-pg20)
"In fact a·(1/a) = 1>0, [and by O5 both factors in the product must be positive since one factor and the product are positive]." (see ita-2-2-13-pg20)
Recall that division, and hence 1/a, was defined in F4. Also, 1/a is called the reciprocal of a.