[Risolto] Non mi vengono...chi mi aiuta? 729 e 731.🙏🏻🙏🏻

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729)

$(3a-2b)(3a+2b)-(6a^3b-4ab^3)÷(-2ab)-6(a+b)(a-b) =$

$= 9a^2-4b^2-(-3a^2+2b^2) -6(a^2-b^2) =$

$= 9a^2-4b^2+3a^2-2b^2-6a^2+6b^2=$

$= (9+3-6)a^2 +(-4-2+6)b^2=$

$= 6a^2+\cancel{0b^2} =$

$= 6a^2$

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731)

$\left(2x-\dfrac{2}{3}y\right)^2-\left[4\left(x+\dfrac{1}{2}y\right)^2-\left(\dfrac{1}{2}x+\dfrac{1}{3}y\right)\left(\dfrac{1}{2}x-\dfrac{1}{3}y\right)\right]+\dfrac{2}{3}\left(\dfrac{1}{2}x+y\right)^2+6xy=$

$=4x^2-\dfrac{8}{3}xy+\dfrac{4}{9}y^2-\left[4\left(x^2+xy+\dfrac{1}{4}y^2\right)-\left(\dfrac{1}{4}x^2-\dfrac{1}{9}y^2\right)\right]+\dfrac{2}{3}\left(\dfrac{1}{4}x^2+xy+y^2\right)+6xy=$

$=4x^2-\dfrac{8}{3}xy+\dfrac{4}{9}y^2-\left[4x^2+4xy+y^2-\dfrac{1}{4}x^2+\dfrac{1}{9}y^2\right]+\dfrac{1}{6}x^2+\dfrac{2}{3}xy+\dfrac{2}{3}y^2+6xy=$

$=\cancel{4x^2}-\dfrac{8}{3}xy+\dfrac{4}{9}y^2\cancel{-4x^2}-4xy-y^2+\dfrac{1}{4}x^2-\dfrac{1}{9}y^2+\dfrac{1}{6}x^2+\dfrac{2}{3}xy+\dfrac{2}{3}y^2+6xy=$

$= \left(-\dfrac{8}{3}-4+\dfrac{2}{3}+6\right)xy +\left(\dfrac{4}{9}-1-\dfrac{1}{9}+\dfrac{2}{3}\right)y^2+\left(\dfrac{1}{4}+\dfrac{1}{6}\right)x^2 =$

$= \left(\dfrac{-8-12+2+18}{3}\right)xy +\left(\dfrac{4-9-1+6}{9}\right)y^2 +\left(\dfrac{3+2}{12}\right)x^2 =$

$= \dfrac{0}{3}xy +\dfrac{0}{9}y^2 +\dfrac{5}{12}x^2 =$

$= \cancel{0xy} \cancel{+0y^2} +\dfrac{5}{12}x^2 =$

$= \dfrac{5}{12}x^2$