20.若a<0,则关于x的不等式x2-4ax-5a2>0的解集是( )
| A. | (-∞,-a)∪(5a,+∞) | B. | (-∞,5a)∪(-a,+∞) | C. | (5a,-a) | D. | (a,-5a) |
19.设a1,a2,…a9成等差数列,若$\sum_{k=1}^{9}{a}_{k}=0,\sum_{k=1}^{9}{a}_{k}^{2}=15$,且a1<a2,则a9=( )
| A. | 2 | B. | $\frac{3}{2}$ | C. | 1 | D. | $\frac{3}{4}$ |
18.设复数z满足1+2z+4z2+8z3=0,则|z|=( )
| A. | 2 | B. | 1 | C. | $\frac{1}{2}$ | D. | $\frac{1}{4}$ |
17.两条直线l1:x-3y+1=0与直线l2:x+2y-5=0的夹角是( )
| A. | $\frac{π}{2}$ | B. | $\frac{π}{4}$ | C. | $\frac{3π}{4}$ | D. | arctan$\sqrt{2}$ |
16.过直线x+y=2与x-y=0的交点,且法向量为$\overrightarrow{n}$=(2,-3)的直线方程是( )
| A. | -3x+2y+1=0 | B. | 3x-2y+1=0 | C. | -2x+3y+1=0 | D. | 2x-3y+1=0 |
14.圆(x+2)2+(y-3)2=7的圆心与半径分别是( )
| A. | (2,-3),7 | B. | (-2,3),7 | C. | (2,-3),$\sqrt{7}$ | D. | (-2,3),$\sqrt{7}$ |
13.若an=1+$\frac{1}{2}$+$\frac{1}{3}$+…+$\frac{1}{{n}^{2}+1}$(n∈N*),则a2等于( )
| A. | 1+$\frac{1}{2}$ | B. | $\frac{1}{5}$ | C. | 1$+\frac{1}{2}$$+\frac{1}{3}$+$\frac{1}{4}$+$\frac{1}{5}$ | D. | 非以上答案 |
12.已知实数x,y满足$\left\{\begin{array}{l}{x≥1}\\{x+y≤5}\\{2x-y≤2}\end{array}\right.$若z=y+mx有最大值12,则实数m的取值为( )
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| A. | -4 | B. | -8 | C. | 8 | D. | 4 |