5sin²x + 3sinx × cosx - 4 = 0
5sin²x + 3sinx × cosx - 4×1 = 0
5sin²x + 3sinx × cosx - 4(sin²x + cos²x) = 0
5sin²x + 3sinx × cosx - 4sin²x - 4cos²x = 0
sin²x + 3sinx × cosx - 4cos²x = 0 | : cos²x
tg²x + 3tgx - 4 = 0
Пусть tgx = a, тогда:
a² + 3a - 4 = 0
D = 3² - 4×1×(-4) = 9 + 16 = 25
D>0, 2 корня
x₁ = -3+√25/2×1 = -3+5/2 = 2/2 = 1
x₂ = -3-√25/2×1 = -3-5/2 = -8/2 = -4
tgx = 1 или tgx = - 4
x₁ = π/4 + πn, n∈Z x₂ = arctg(-4) + πn, n∈Z
x₂ = - arctg 4 + πn, n∈Z
ответ: x₁ = π/4 + πn, n∈Z
x₂ = - arctg 4 + πn, n∈Z
