13.下列变形为因式分解的是( )
| A. | $\frac{1}{{x}^{2}}$-$\frac{1}{{y}^{2}}$=($\frac{1}{x}$+$\frac{1}{y}$)($\frac{1}{x}$-$\frac{1}{y}$) | B. | ($\sqrt{x}$+$\sqrt{y}$)($\sqrt{x}$-$\sqrt{y}$)=x-y | ||
| C. | (x-2y)(x+2y)=x2-4y2 | D. | (a+b)2-(a-b)2=4ab |
12.已知x=$\frac{\sqrt{3}-\sqrt{2}}{\sqrt{3}+\sqrt{2}}$,y=$\frac{\sqrt{3}+\sqrt{2}}{\sqrt{3}-\sqrt{2}}$,则3x2-5xy+3y2的值为( )
| A. | 290 | B. | 289 | C. | 288 | D. | 287 |
10.如果一个三角形的三个外角的度数之比是2:3:4,那么与之对应的三个内角的度数之比是( )
0 286651 286659 286665 286669 286675 286677 286681 286687 286689 286695 286701 286705 286707 286711 286717 286719 286725 286729 286731 286735 286737 286741 286743 286745 286746 286747 286749 286750 286751 286753 286755 286759 286761 286765 286767 286771 286777 286779 286785 286789 286791 286795 286801 286807 286809 286815 286819 286821 286827 286831 286837 286845 366461
| A. | 1:3:5 | B. | 2:3:4 | C. | 4:3:2 | D. | 5:3:1 |