1+sina
=sin²(a/2)+cos²(a/2)+2sin(a/2)cos(a/2)
=[sin(a/2)+cos(a/2)]²
=[√2sin(a/2+π/4)]²
=2sin²(a/2+π/4)
5π/2≤a≤7π/2
5π/4≤a/2≤7π/4
3π/2≤a/2+π/4≤2π
所以sin(a/2+π/4)