15.函数f(x)=$\left\{\begin{array}{l}{f(x+2),x<2}\\{{2}^{-x},x≥2}\end{array}\right.$,则f(0)=( )
A. | 4 | B. | 8 | C. | $\frac{1}{8}$ | D. | $\frac{1}{4}$ |
10.在平行四边形ABCD中,$\overrightarrow{AB}$=(cos18°,cos72°),$\overrightarrow{BC}$=(2cos63°,2cos27°),则四边形ABCD的面积为( )
A. | $\frac{\sqrt{2}}{4}$ | B. | $\sqrt{2}$ | C. | $\frac{\sqrt{3}}{2}$ | D. | $\frac{\sqrt{2}}{2}$ |
9.已知数列{an}是公比为2的等比数列,若a1=2.则$\frac{1}{{a}_{1}^{2}}$+$\frac{1}{{a}_{2}^{2}}$+…+$\frac{1}{{a}_{n}^{2}}$=( )
0 249945 249953 249959 249963 249969 249971 249975 249981 249983 249989 249995 249999 250001 250005 250011 250013 250019 250023 250025 250029 250031 250035 250037 250039 250040 250041 250043 250044 250045 250047 250049 250053 250055 250059 250061 250065 250071 250073 250079 250083 250085 250089 250095 250101 250103 250109 250113 250115 250121 250125 250131 250139 266669
A. | $\frac{1}{3}$(1-$\frac{1}{{2}^{n}}$) | B. | $\frac{1}{3}$(4n-1) | C. | $\frac{1}{3}$(1-$\frac{1}{{4}^{n}}$) | D. | 1-$\frac{1}{{4}^{n}}$ |