Как я понимаю, это прямоугольный треугольник?
2) tg A = BC/AC; BC = AC*tg A
sin A = 6√61/61 = 6/√61;
cos^2 A = 1 - sin^2 A = 1 - 36/61 = 25/61
cos A = 5/√61
tg A = sin A / cos A = 6/5
BC = AC*tg A = 4*6/5 = 24/5 = 4,8
3) tg A = BC/AC; BC = AC*tg A
cos A = 3√109/109 = 3/√109;
sin^2 A = 1 - cos^2 A = 1 - 9/109 = 100/109
sin A = 10/√109
tg A = sin A / cos A = 10/3
BC = AC*tg A = 1,5*10/3 = 15/3 = 5
5) tg A = 15/8
1/cos^2 A = 1 + tg^2 A = 1 + 225/64 = 289/64
cos^2 A = 64/289; cos A = 8/17
sin A = tg A * cos A = 15/8*8/17 = 15/17
BC = AB*sin A = 13,6*15/17 = 0,8*17*15/17 = 0.8*15 = 12