9.(苏皖学校高三第三次月考数学试卷)

已知二次函数f(x)=ax2+bx+c(a,b,c均为实数),满足a-b+c=0,对于任意实数x 都有f (x)-x≥0,并且当x∈(0,2)时,有f (x)≤.

  (1)求f (1)的值;

(2)证明:ac

(3)当x∈[-2,2]且a+c取得最小值时,函数F(x)=f (x)-mx (m为实数)是单调的,求证:mm.

解:(1)∵对于任意xR,都有f (x)-x≥0,且当x∈(0,2)时,

f (x) ≤.令x=1

∴1≤f (1) ≤.

f (1)=1.···················································································· 5分

  (2) 由a-b+c=0及f (1)=1.

,可得b=a+c=.···················································· 7分

又对任意x,f(x)-x≥0,即ax2-x+c≥0.

a>0且△≤0.

-4ac≤0,解得ac.·························································· 9分

(3) 由(2)可知a>0,c>0.

a+c≥2≥2·=.························································ 10分

当且仅当时等号成立.此时

a=c=.·······················································································

f (x)= x2+x+,

F (x)=f (x)-mx=[x2+(2-4m)x+1].····················································· 12分

x∈[-2,2]时,f (x)是单调的,所以F (x)的顶点一定在[-2,2]的外边.

≥2.············································································ 13分

解得m≤-m. …………………………………………………………..14分

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