Question

Se cos x= 1/3, quanto vale cos (2x)?

Answer 1

cosx = 1/3
sex = ?

cos²x + sen²x= 1
1/9 + sen²x = 
sen²x = 9/9 - 1/9 = 8/9
senx =+ 2√2/3 

cos (x x) = cosx.cosx - senx.senx = 1/9 - 8/9 = -7/9