4.已知$\left\{\begin{array}{l}{x=2}\\{y=1}\end{array}\right.$是二元一次方程组$\left\{\begin{array}{l}{mx+ny=7}\\{nx-my=1}\end{array}\right.$的解,则$\root{3}{m+3n}$的值为( )
| A. | 3 | B. | 8 | C. | 2 | D. | $\sqrt{2}$ |
2.若$\sqrt{x-\frac{1}{8}}$+$\sqrt{\frac{1}{8}-x}$有意义,则$\root{3}{x}$=( )
0 296436 296444 296450 296454 296460 296462 296466 296472 296474 296480 296486 296490 296492 296496 296502 296504 296510 296514 296516 296520 296522 296526 296528 296530 296531 296532 296534 296535 296536 296538 296540 296544 296546 296550 296552 296556 296562 296564 296570 296574 296576 296580 296586 296592 296594 296600 296604 296606 296612 296616 296622 296630 366461
| A. | -$\frac{1}{2}$ | B. | $\frac{1}{8}$ | C. | $\frac{1}{2}$ | D. | -$\frac{1}{8}$ |