sin x - sin y = 2 sin( (x - y)/2 ) cos( (x + y)/2 )
cos(3x+a) = sin (pi/2 - 3x - a)
f(x) = 2 sin(3x+a-pi/4) cos(pi/4)
a-pi/4 = pi
a = 5pi/4f(x) = 2 sin(3x+a-pi/4) cos(pi/4) 是怎么的出来的?套用公式sin x - sin y = 2 sin( (x - y)/2 ) cos( (x + y)/2 )