u=asinb+asina+bsina+bsinb
=a(sinb+sina)+b(sinb+sina)
=(a+b)(sina+sinb)
a+b=π/2 sinb=cosa
u=π/2(sina+cosa)=根2π/2 sin(a+π/4)
当a=π/4时
最大值是根2*π/2