8.已知a+b(a>0,b>0)是函数f(x)=-x+30-3a的零点,则使得$\frac{1}{a}+\frac{1}{b}$取得最小值的有序实数对(a,b)是 ( )
| A. | (10,5) | B. | (7,2) | C. | (6,6) | D. | (5,10) |
4.已知i为虚数单位,复数z=1+2i,z与$\overline z$共轭,则$z\overline z$等于( )
| A. | 3 | B. | $\sqrt{3}$ | C. | $\sqrt{5}$ | D. | 5 |
3.设函数$f(x)=\left\{\begin{array}{l}{x^2}-6x+6,x≥0\\ 3x+4,x<0\end{array}\right.$,若互不相等的实数x1,x2,x3满足f(x1)=f(x2)=f(x3),则x1+x2+x3的取值范围是( )
0 227228 227236 227242 227246 227252 227254 227258 227264 227266 227272 227278 227282 227284 227288 227294 227296 227302 227306 227308 227312 227314 227318 227320 227322 227323 227324 227326 227327 227328 227330 227332 227336 227338 227342 227344 227348 227354 227356 227362 227366 227368 227372 227378 227384 227386 227392 227396 227398 227404 227408 227414 227422 266669
| A. | $({\frac{11}{6},6}]$ | B. | $({\frac{11}{3},6})$ | C. | $({\frac{20}{3},\frac{26}{3}})$ | D. | $({\frac{20}{3},\frac{26}{3}}]$ |