a+b=ba- 1

ba- 1≥2ab

-1≥ab (because ab is positive, you can multiply and then move the term) ①

3a+2b≥2*3a*2b

3a+2b≥ 12ab ②

Look at the simultaneous expressions of formula 1 and formula 2. The maximum value of ab is-1 and the minimum value is-12.