a²+2a+4b²-4b+2=0
(a²+2a+1)+(4b²-4b+1)=0
(a+1)²+(2b-1)²=0
a+1=0 且 2b-1=0
a=-1,b=1/2
原式=(a+b)²+(a-b)(2a+b)-3a²
=a²+2ab+b²+a×2a+a×b-b×2a-b×b-3a²
=a²+2ab+b²+2a²+ab-2ab-b²-3a²
=(a²+2a²-3a²)+(2ab+ab-2ab)+(b²-b²)
=ab
=-1×1/2
=-1/2