5. Calculaţi:
a) cos(x − y), dacă x, y € (0, π/2) şi cos x = 1/7, cos y = 13/14

b) cos(π/3 - a), daca a € (3 π/2, π) si sin a = -12/-13

c) sin(a - b) dacă a € (0; π/2), b € (3 π/2; 2 π) si sin a = √2/2, sin b = (√2-√6)/4

Am nevoie urgent, dau coroana