18.已知$\sqrt{a-b+4}$+$\sqrt{a+b}$=0,则a2的值为( )
| A. | 4 | B. | 1 | C. | 0 | D. | -4 |
15.已知a2=25,$\sqrt{{b}^{2}}$=7,且|a+b|=a+b,则a-b的值为( )
| A. | 2或12 | B. | 2或-12 | C. | -2或12 | D. | -2或-12 |
12.下列计算正确的是( )
0 283592 283600 283606 283610 283616 283618 283622 283628 283630 283636 283642 283646 283648 283652 283658 283660 283666 283670 283672 283676 283678 283682 283684 283686 283687 283688 283690 283691 283692 283694 283696 283700 283702 283706 283708 283712 283718 283720 283726 283730 283732 283736 283742 283748 283750 283756 283760 283762 283768 283772 283778 283786 366461
| A. | (-4x)•(2x2+3x-1)=-8x3-12x2-4x | |
| B. | (6xy2-4x2y)•3xy=6xy2-12x3y2 | |
| C. | (-x)•(2x+x2-1)=-x3-2x2+1 | |
| D. | (-3x2y)(-2xy+3yz+1)=6x3y2-9x2y2z-3x2y |