c-b=b-a=π/8
得b=a+(π/8),c=b+(π/8)=a+(π/8)+(π/8)=a+(π/4)
f(a)+f(b)+f(c)
=2a-cosa+2[a+(π/8)]-cos[a+(π/8)]+2[a+(π/4)]-cos[a+(π/4)]
=6a+(3π/4)-[cosa +cos(a+ π/8)+cos(a+ π/4)]
=6a+(3π/4)-[2cos(-π/16)cos(a+ π/4)+cos(a+ π/4)]
所以 6a-[cosa +cos(a+ π/8)+cos(a+ π/4)]=9π/4
因是选择题,将ABCD依次带入上式,经验证,
a=3π/8,符合上式
所以 答案选B