y = sqrt(13) sin(2wx+A) - sqrt(3)
2w(x1-x2) = 2pi,A = arctan(2sqrt(3))
w = 2
f(a) = sqrt(13)sin(4a+A) -sqrt(3) = 2/3
4a + A =arcsin[ (2/3+sqrt(3))/sqrt(13)] = B
sin(5pi/6 - 4a) = sin(5pi/6 + A - arcsin(...) = sin5pi/6 cos(A-B) + cos5pi/6 sin(A-B)
= 1/2 [cosAcosB+sinAsinB -sinAcosB + cosAsinB]
= ...
= 0.5469
(你可以推导出精确值)