Найти В используя данные

∠В = 12,68°*2 = 25,36°

18² + X² = 4 · 41²
324 + X² = 4 · 1681
X² = 6724 − 324 = 6400
X = 80

41² = 41² + 80² − 2 · 41 · 80 · cos(B/2)
1681 = 1681 + 6400 − 6560 · cos(B/2)
6560 · cos(B/2) = 6400
cos(B/2) = 6400 / 6560 = 40/41

      40/41 ≈ 0,9756  ⇒  B/2 ≈ arccos(0,9756) ≈ 12,7°
      B = 2 · 12,7° ≈ 25,4°