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\begin{aligned} f(2) &=2^{2}-\dfrac{1}{2} & f(4) &=4^{2}-\dfrac{1}{4} \\ &=4-\dfrac{1}{2} & &=16-\dfrac{1}{4} \\ &=\dfrac{7}{2} & &=\dfrac{63}{4} \end{aligned}