sin(30+a) = sin(π/6 + a) = sin(π/2 + a - π/3) = -cos(π/3 + a) = -cos(60+a) = -cos(60)cos(a) + sin(60)sin(a) = -1/2cos(a) + √3/2sin(a) = √3/2sin(a) - 1/2cos(a)