(questo è l’ultimo dei tre esercizi)
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$\small = 9x^2\left(\dfrac{2}{3}a-x\right)\left(\dfrac{2}{3}a+x\right)-9\left(x^2+2\right)\left(-x^2-2\right)-\left(1+2ax\right)\left(-1+2ax\right) =$
$\small = 9x^2\left(\dfrac{4}{9}a^2-x^2\right)-9\left(-x^4-2x^2-2x^2-4\right)-\left(-1\cancel{+2ax}\cancel{-2ax}+4a^2x^2\right) =$
$\small = \dfrac{\cancel{36}^4}{\cancel9_1}a^2x^2-9x^4-9\left(-x^4-4x^2-4\right)+1-4a^2x^2 =$
$\small = \cancel{4a^2x^2}\cancel{-9x^4}\cancel{+9x^4}+36x^2+36+1\cancel{-4a^2x^2} =$
$\small = 36x^2+36+1=$
$\small = 36x^2+37$