=15a^2b-5ab^2-ab^2-3a^2b =12a^2b-6ab^2 =12*1/2*1/2*1/3-6*1/2*1/3*1/3 =2/3
=15a^2b-5ab^2-ab^2-3a^2b=12a^2b-6ab^2=12*1/2*1/2*1/3-6*1/2*1/3*1/3=2/3
