7.若曲线y=$\sqrt{9-{x}^{2}}$与直线y=2x+b始终有交点,则b的取值范围是( )
A. | [-6,3$\sqrt{2}$] | B. | [-6,3$\sqrt{5}$] | C. | [-3$\sqrt{5}$,3$\sqrt{5}$] | D. | [-3$\sqrt{2}$,3$\sqrt{2}$] |
6.已知0<x<1,x2-3x+1=0,则x${\;}^{\frac{1}{2}}$-x${\;}^{-\frac{1}{2}}$的值为( )
A. | 1 | B. | -1 | C. | 1或-1 | D. | -$\sqrt{5}$ |
5.下列函数在(0,+∞)为减函数的是( )
0 250162 250170 250176 250180 250186 250188 250192 250198 250200 250206 250212 250216 250218 250222 250228 250230 250236 250240 250242 250246 250248 250252 250254 250256 250257 250258 250260 250261 250262 250264 250266 250270 250272 250276 250278 250282 250288 250290 250296 250300 250302 250306 250312 250318 250320 250326 250330 250332 250338 250342 250348 250356 266669
A. | y=-|x-1| | B. | y=|x2-4| | C. | y=-$\frac{3}{x}$ | D. | y=-x(x+2) |