3.若f(x+$\frac{1}{x}$)=x2+$\frac{1}{{x}^{2}}$,则f(x)=( )
A. | x2-2 | B. | x2+$\frac{1}{{x}^{2}}$ | C. | x2+2 | D. | x2-$\frac{1}{{x}^{2}}$ |
20.已知f(x)=$\left\{\begin{array}{l}{-x,x>0}\\{{x}^{2},x<0}\end{array}\right.$,则f[f(x)]=( )
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A. | $\left\{\begin{array}{l}{{x}^{2},x>0}\\{-{x}^{2},x<0}\end{array}\right.$ | B. | $\left\{\begin{array}{l}{-{x}^{2},x>0}\\{{x}^{2},x<0}\end{array}\right.$ | ||
C. | $\left\{\begin{array}{l}{-x,x>0}\\{{x}^{2},x<0}\end{array}\right.$ | D. | $\left\{\begin{array}{l}{-x,x<0}\\{{x}^{2},x>0}\end{array}\right.$ |