山西省晋中市2023-2024学年第一学期九年级12月教学水平调研卷数学
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山西省晋中市2023-2024学年第一学期九年级12月教学水平调研卷数学试卷答案
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分析(1)先化简函数式,将函数化为sinx的二次型函数,再用分离参数法和单调性求解;
(2)讨论二次函数在“动轴定区间”上的最值,再列方程求解.
解答解:(1)当b=1时,函数式可化简如下:
f(x)=$\frac{1}{2}$(cosx-sinx)•(cosx+sinx)-2asinx+1
=$\frac{1}{2}$(cos2x-sin2x)-2asinx+1=-sin2x-2asinx+$\frac{3}{2}$,
令t=sinx(0<t<$\frac{1}{2}$),对任意x∈(0,$\frac{π}{6}$),恒有f(x)>0,
即为-t2-2at+$\frac{3}{2}$>0,分离参数得:-2a>t-$\frac{3}{2t}$,
由t-$\frac{3}{2t}$在(0,$\frac{1}{2}$)递增,所以,t-$\frac{3}{2t}$<$\frac{1}{2}$-3=-$\frac{5}{2}$,
因此,-2a>-$\frac{5}{2}$,解得,0<a<$\frac{5}{4}$,
即实数a的取值范围为(0,$\frac{5}{4}$);
(2)f(x)=-sin2x-2asinx+b+$\frac{1}{2}$,令t=sinx(-1≤t≤1),
记g(t)=-t2-2at+b+$\frac{1}{2}$,图象的对称轴t=-a<0,且开口向下,
①当-a≤-1时,即a≥1,函数g(t)在[-1,1]上单调递减,则
g(t)max=g(-1)=-1+2a+b+$\frac{1}{2}$=1,
g(t)min=g(1)=-1-2a+b+$\frac{1}{2}$=-4,
解得a=$\frac{5}{4}$,b=-1;
②当-1<-a<1时,即0<a<1,函数g(t)在[-1,1]上先增后减,则
g(x)max=g(-a)=$\frac{1}{2}$+b+a2=1,
g(x)min=g(1)=-1-2a+b+$\frac{1}{2}$=-4,
解方程可得a=$\sqrt{5}$-1,b=2$\sqrt{5}$-$\frac{11}{2}$,由于a=$\sqrt{5}$-1>1,不合题意,舍去.
综上可得a=$\frac{5}{4}$,b=-1.
点评本题主要考查三角函数的化简和求值,以及不等式恒成立问题的解法,运用了参数分离和函数的单调性,属于中档题.