15.△ABC中,a,b,c分别是角A,B,C的对边,且sinA+cosA=$\frac{\sqrt{3}-1}{2}$,a=7,3sinB=5sinC,则b+c的值为( )
A. | 12 | B. | 8$\sqrt{3}$ | C. | 8$\sqrt{2}$ | D. | 8 |
12.定义在R上的函数y=f(x),满足f(x+2)=-$\frac{1}{f(x)}$,则( )
0 251744 251752 251758 251762 251768 251770 251774 251780 251782 251788 251794 251798 251800 251804 251810 251812 251818 251822 251824 251828 251830 251834 251836 251838 251839 251840 251842 251843 251844 251846 251848 251852 251854 251858 251860 251864 251870 251872 251878 251882 251884 251888 251894 251900 251902 251908 251912 251914 251920 251924 251930 251938 266669
A. | f(x)不是周期函数 | B. | f(x)是周期函数,且最小正周期为2 | ||
C. | f(x)是周期函数,且最小正周期为4 | D. | f(x)是周期函数,且4是它的一个周期 |