MathematicsMedium179×since 2002Q4246Let f(x)={x−1,x is even, 2x,x is odd, x∈Nf(x)=\left\{\begin{array}{l}x-1, x \text { is even, } \\ 2 x, \quad x \text { is odd, }\end{array} x \in \mathbf{N}\right.f(x)={x−1,x is even, 2x,x is odd, x∈N. If for some a∈N,f(f(f(a)))=21\mathrm{a} \in \mathbf{N}, f(f(f(\mathrm{a})))=21a∈N,f(f(f(a)))=21, then limx→a−{∣x∣3a−[xa]}\lim\limits_{x \rightarrow \mathrm{a}^{-}}\left\{\frac{|x|^3}{\mathrm{a}}-\left[\frac{x}{\mathrm{a}}\right]\right\}x→a−lim{a∣x∣3−[ax]}, where [t][t][t] denotes the greatest integer less than or equal to ttt, is equal to :A169B121C225D144Check answerSkip