解
将x=2,y=1代入得:
2a+b=7 ①
2b+a=1 ②
①+②得:
3a+3b=8
a+b=8/3
0.5a^2+ab+0.5b^2
=0.5(a^2+2ab+b^2)
=0.5(a+b)^2
=1/2×64/9
=32/9