1. а)sin(5π/14 + π/7) = sin(7π/14) = sin(π/2) = 1

2. a) cos(78°-18°) = cos(60°) = 1/2

2. sin(a)cos(b) - sin(a-b) = sin(a)cos(b) - sin(a)cos(b) + sin(b)cos(a) = sin(b)cos(a)

б) cos(π/3)cos(x) - sin(π/3)sin(x) + ✓3/2sinx = 1/2cosx - ✓3/2 sinx + √3/2sinx = 1/2cos x

3. cos(a)cos(b)-sin(a)sin(b)-cos(a)cos(b)-sin(a)cos(b) = -2sin(a)sin(b)

Тождество доказано