x² + 2020²/x² - 2021 = x² - 2*x*2020/x + 2020²/x² + 2019 = (x - 2020/x)² + 2019
x ≠ 0
f(0) - не определено
f(x - 2020/x) = x² + 2020²/x² - 2021 = (x - 2020/x)² + 2019
x - 2020/x = t
f(t) = t² + 2019
f(x) = x² + 2019
