Введём дополнительный угол.
Посчитаем C = √(a² + b²)
C = √(1² + 1²) = √2.
Разделим на √2:
sin5x·√2/2 + cos5x·√2/2 = √2/2
sin(π/4) = cos(π/4) = √2/2
cos5x·cos(π/4) + sin5x·sin(π/4) = √2/2
cos(5x - π/4) = √2/2
5x - π/4 = ±π/4 + 2πn, n ∈ Z
5x = ±π/4 + π/4 + 2πn, n ∈ Z
x = ±π/20 + π/20 + 2πn/5, n ∈ Z
Ответ: x = ±π/20 + π/20 + 2πn/5, n ∈ Z.