a^2+ab+b^2-a-2b
=(a+b/2)^2+3/4*b^2-a-2b
=(a+b/2)^2-(a+b/2)+1/4-1/4+b/2+3/4*b^2-2b
=(a+b/2-1/2)^2+(3/4*b^2-3b)-1/4
=(a+b/2-1/2)^2+3/4*(b^2-2b+1)-3/4-1/4
=(a+b/2-1/2)^2+3/4(b-1)^2-1
>=1
當(dāng)b-1=且a+b/2-1/2=0,即b=1,a=0時(shí)取得等號(hào)
即原式的最小值是-1
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