1) tg a = -24/7, тогда cos^2 a = 1/(1+tg^2 a) = 1/(1+576/49) = 49/625. Тангенс отрицательный, значит, угол лежит во 2 четверти, cos a < 0. cos a = -7/25; sin^2 a = 1 - cos^2 a = 1 - 49/625 = 576/625; sin a = 24/25. tg B = 15/5 = 3, cos^2 a = 1/(1+9) = 1/10; cos a = 1/√10. sin^2 a = 1 - cos^2 a = 1 - 1/10 = 9/10; sin a = 3/√10. cos(a+b) = cos a*cos b-sin a*sin b = -7/25*1/√10-24/25*3/√10 = -79/(25√10)
Тангенс отрицательный, значит, угол лежит во 2 четверти, cos a < 0.
cos a = -7/25; sin^2 a = 1 - cos^2 a = 1 - 49/625 = 576/625; sin a = 24/25.
tg B = 15/5 = 3, cos^2 a = 1/(1+9) = 1/10; cos a = 1/√10.
sin^2 a = 1 - cos^2 a = 1 - 1/10 = 9/10; sin a = 3/√10.
cos(a+b) = cos a*cos b-sin a*sin b = -7/25*1/√10-24/25*3/√10 = -79/(25√10)
2) 1/2*arcsin(√3/2) - 2*arccos(-1/2) = 1/2*pi/3 - 2*2pi/3 = pi/6 - 4pi/3 = -7pi/6