Question

sabendo que cos de 12π/7 =m , determine : a) o sinal de m. b) o valor de cos 9π/7 em função de m

Answer 1

Ola André 

m = cos(12π/7) = +0.6234898

cos(a) + cos(b) = 2*cos[(a+b)/2]*cos[(a-b)/2]

cos(12π/7) + cos(9π/7) = 2*cos(21π/14)*cos(3π/14) 

cos(21π/14) = cos(3π/2) = 0

2*cos(21π/14)*cos(3π/14) = 0

m + cos(9π/7) = 0

cos(9π/7) = -m