a(a²+2b)+b(-3a+b²)=0.5=a³+2ab-3ab+b³=0.5=a³+b³-ab=0.5=（a+b)(a²-ab+b²）-ab=0.5∵a+b=1∴a²-ab+b²-ab=0.5=a²-2ab+b²=0.5=（a+b）²-2ab-2ab=0.5（a+b）²-4ab=0.5∵a+b=1∴1-4ab=0.5-4ab=-0.5ab=1/8
