第1个回答 2013-05-20
设b=c。∴a�0�5+2b�0�5=a�0�5+b�0�5+c�0�5=1。∴a�0�5=1-2b�0�5。
设y=ab+bc+ca=2ab+b�0�5。y�0�5-2b�0�5y+b^4=(y-b�0�5)�0�5=(2ab)�0�5=4a�0�5b�0�5=4(1-2b�0�5)b�0�5。9b^4-(2y+4)b�0�5+y�0�5=0
0≤⊿=(2y+4)�0�5-4*9*y�0�5=-32y�0�5+16y+16=-16(2y�0�5-y-1)=-16(2y+1)(y-1)。∴N=-1/2≤y≤1=M。∴MN=-1/2