12.在某次射击训练中,甲、乙、丙、丁4人各射击10次,平均成绩相同,方差分别是S甲2=0.25,S乙2=0.35,S丙2=0.19,S丁2=0.28,这4人中成绩发挥最稳定的是( )
| A. | 甲 | B. | 乙 | C. | 丙 | D. | 丁 |
7.下列方程变形中,正确的是( )
0 297468 297476 297482 297486 297492 297494 297498 297504 297506 297512 297518 297522 297524 297528 297534 297536 297542 297546 297548 297552 297554 297558 297560 297562 297563 297564 297566 297567 297568 297570 297572 297576 297578 297582 297584 297588 297594 297596 297602 297606 297608 297612 297618 297624 297626 297632 297636 297638 297644 297648 297654 297662 366461
| A. | 方程3x=2,得x=$\frac{3}{2}$ | |
| B. | 方程3-x=2-5(x-1),去括号,得3-x=2-5x-1 | |
| C. | 方程3x-2=2x+1,移项,得3x-2x=-1+2 | |
| D. | 方程$\frac{x-1}{0.2}$-$\frac{x}{0.5}$=1,化成5x-5-2x=1 |