19.若y=(1+m)${x}^{{m}^{2}-7}$是二次函数,且开口向下,则m的值为( )
| A. | ±3 | B. | -3 | C. | +3 | D. | 0 |
18.下列关系式中,属于二次函数的是(x为自变量)( )
0 279654 279662 279668 279672 279678 279680 279684 279690 279692 279698 279704 279708 279710 279714 279720 279722 279728 279732 279734 279738 279740 279744 279746 279748 279749 279750 279752 279753 279754 279756 279758 279762 279764 279768 279770 279774 279780 279782 279788 279792 279794 279798 279804 279810 279812 279818 279822 279824 279830 279834 279840 279848 366461
| A. | y=$\frac{1}{8}{x^2}$ | B. | y=$\sqrt{{x^2}-1}$ | C. | y=$\frac{1}{x^2}$ | D. | y=$\frac{1}{x}$ |