14.已知f(x)=$\left\{\begin{array}{l}{0,x>0}\\{1,x=0}\\{2x-1,x<0}\end{array}\right.$,则f(f[f(6)])的值是( )
| A. | 0 | B. | 1 | C. | -1 | D. | 3 |
12.若sinα+cosα=$\frac{\sqrt{5}}{5}$(α是第二象限角),则tanα的值是( )
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| A. | -2 | B. | -$\frac{1}{2}$ | C. | $\frac{4}{3}$ | D. | $\frac{2}{5}$ |