Cách làm cho bạn:
a. $x^{2}.(5x^{3}-x-\frac{1}{2})$
= $x^{2}.5x^{3}-x^{2}.x-x^{2}.-\frac{1}{2}$
= $5x^{5}– x^{3}– 1212x^{2}$
b. $(3xy-x^{2}+y)\frac{2}{3}x^{2}y$
= $3xy.\frac{2}{3}x^{2}y-x^{2}.\frac{2}{3}x^{2}y+y.\frac{2}{3}x^{2}y$
= $2x^{3}2y^{2}– 2323x^{4}y +2323x^{2}y^{2}$
c. $(4x^{3}-5xy+2x)(-\frac{1}{2}xy)$
= $4x^{3}.(-\frac{1}{2}xy)-5xy.(-\frac{1}{2}xy)+2x.(-\frac{1}{2})$
= $-2x^{4}y + 5252x^{2}y^{2}-x^{2}y.$
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