Question

dada sen a=$\frac{ \sqrt{3} }{3}$ , calcule cos (2a)

Answer 1

cos2a = cos( a + a) = cos²a - sen²a = 1 - sen²a - sen²a = 1 - 2sen²a

= 1 - 2(√3/3)² = 1 - 2.3/9 = 1 - 2.1/3 = 1 - 2/3 = (3 - 2)/3 = 1/3