18.在四面体ABCD中,AB=3,BC=7,CD=11,DA=9.则$\overrightarrow{AC}$•$\overrightarrow{BD}$的值为( )
A. | 0 | B. | 1 | C. | 2 | D. | $\frac{1}{2}$ |
17.已知函数f(x)在R上单调递增,当x1+x2=1时,恒有f(x1)+f(0)>f(x2)+f(1),则x1的取值范围是( )
A. | (-∞,0) | B. | (0,$\frac{1}{2}$) | C. | ($\frac{1}{2}$,1) | D. | (1,+∞) |
12.已知f(x)、g(x)、h(x)均为一次函数.若对实数x满足:
|f(x)|-|g(x)|+h(x)=$\left\{\begin{array}{l}{-2,x<-1}\\{7x+5,-1≤x<0}\\{-4x+5,x≥0}\end{array}\right.$,h(x)的解析式为.
|f(x)|-|g(x)|+h(x)=$\left\{\begin{array}{l}{-2,x<-1}\\{7x+5,-1≤x<0}\\{-4x+5,x≥0}\end{array}\right.$,h(x)的解析式为.
A. | 2x-$\frac{3}{2}$ | B. | -2x-$\frac{3}{2}$ | C. | 2x+$\frac{3}{2}$ | D. | -2x+$\frac{3}{2}$ |
11.已知圆x2+y2-4x+2y-11=0的一条直径过直线x-2y-3=0被圆截得的弦的中点,则该直径所在的直线方程是( )
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A. | 2x+y-5=0 | B. | x-2y=0 | C. | 2x+y-3=0 | D. | x+2y=0 |