По теореме косинусов
а² = m² + n² - 2m·n·cos30° = 1 + 3 - 2·1·√3 ·0.5√3 = 1 + 3 - 3 = 1
поскольку IaI = ImI, то ∠(а, m) = 180° - 2·30° = 120°
b² = m² + 4n² + 2m·2n·cos30° = 1 + 12 + 2·1·2√3·0.5√3 = 19
4n² = b² + m² - 2m·b· cos(m,b)
12 = 19 + 1 - 2·1·√19 ·cos(m,b)
cos(m,b) = 8 : 2√19 = 0.9177
∠(m,b) = 23.4°
∠(a,b) = 120° + 23.4 = 143.4°