19.若a,b,c>0,且$a(a+b+c)+bc=4+2\sqrt{3}$,则2a+b+c的最小值为( )
| A. | $\sqrt{3}-1$ | B. | $2\sqrt{3}+2$ | C. | $\sqrt{3}+1$ | D. | $2\sqrt{3}-2$ |
18.已知等比数列{an}的各项均为正数,公比0<q<1,设P=$\frac{{a}_{3}+{a}_{9}}{2}$,Q=$\sqrt{{a}_{5}{a}_{7}}$,则a3,a9,P与Q的大小关系是( )
| A. | a3>P>Q>a9 | B. | a3>Q>P>a9 | C. | a9>P>a3>Q | D. | P>Q>a3>a9 |
17.在等比数列{an}中,a1=2,an+1=3an,则其前n项和为Sn的值为( )
| A. | 3n-1 | B. | 1-3n | C. | $\frac{1}{{{3^{n-1}}}}-1$ | D. | $1-\frac{1}{{{3^{n-1}}}}$ |
16.已知△ABC中,a,b,c分别为角A,B,C的对应边,A=30°,B=45°,a=7,则边长b为( )
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| A. | $\frac{7}{2}\sqrt{2}$ | B. | $14\sqrt{2}$ | C. | $7\sqrt{2}$ | D. | $\frac{7}{3}\sqrt{6}$ |