20d−2020d-2020d−2020+20d20+20d20+20d =20d+2020d+2020d+2020d−2020d-2020d−20 =
= 20d20d20d² - 20² = 400d² -400
20d−2020d-2020d−2020+20d20+20d20+20d =20d+2020d+2020d+2020d−2020d-2020d−20 =
= 20d20d20d² - 20² = 400d² -400