he sum and difference formulas are in section 5 2 of your textbook Example Find sin 105 According to section 5 2 of our textbook sin a sin a cos 6 cos a sin We know 150 45 105 so we can use a 150 and 3 45 We chose those two because we already know the sines and cosines of those two angles sin 105 sin 150 45 sin 150 cos 45 cos 1 3 1 2 2 2 2 2 6 2 4 6 4