2cos5αcos8α=cos13α;
cos(5α + 8α) + cos(5α - 8α) = cos13α;
cos13α + cos3α = cos13α;
cos3α = 0
3α = π/2 + πn, n ∈ Z
α = π/6 + πn/3, n ∈ Z
ответ: π/6 + πn/3, n ∈ Z.
