Tìm x biết

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a/ \( x^{m-1} \cdot (x + x^{2-m}) \)
b/ \( 2x^{3m+1} \cdot (x^{4-2m} + 3) \)
c/ \( \left( \frac{x^2 + \frac{1}{2}x^{2+k+1}}{2} \right) \cdot x^{k-2} + 2x^{1-k} \)
d/ \( \left( \frac{2}{3}y^3 + 6y^{2k-1} \right) \left( y^{k-1} - \frac{1}{9}y^{3-k} \right) \)