(1) f(-x) = sin a cos x - (tan a - 2 )sinx - sin a = f(x),则 tan a - 2 = 0,那么tan a = 2
(2) f(x) = sin a cos x - sin a = sin a (cos x - 1)
f (x)>= 0 ,则 sin a < 0 ,f(x)取最大值时 cos x = 0
x = PIE*(1/2 + k ),其中k为整数.