a(a^2+2b)+b(-3a+b^2)
=a^3+2ab-3ab+b^3
=(a^3+b^3)-ab
=(a+b)(a^2-ab+b^2)-ab
=(a+b)[(a+b)^2-3ab]-ab
a+b=1
原式=(1-3ab)-ab=1-4ab=0.5
4ab=0.5
ab=1/8