cos(a-π/4) = 1/4 ,cosa*cos(π/4)+sina*sin(π/4) = 1/4 ,(√2/2)(cosa+sina) = 1/4 ,(√2/2)²(cosa+sina)² = (1/4)² ,(1/2)(cos²a+sin²a+2sinacosa) = 1/16 ,(1/2)(1+sin2a) = 1/16 ,1+...