sin2α = 2sinα × cosα
cos2α = cos²α - sin²α
cos2α = 2cos²α - 1
cos2α = 1 - 2sin²α
tg2α =
ctg2α =
x² + 4x - 2 = (x² + 4x + 4) - 4 - 2 = (x + 2)² - 6 = (x + 2)² - (√6)² =
= (x + 2 + √6)(x + 2 - √6)