|a|²=7cos²C/2+cos²(A-B)/2
=7×(1+cosC)/2+[1+cos(A-B)]/2
=4+[7cosC+cos(A-B)]/2
=4+[-7cos(A+B)+cos(A-B)]/2
=4+4sinAsinB-3cosAcosB
=4
∴|a|=2
|a|²=7cos²C/2+cos²(A-B)/2
=7×(1+cosC)/2+[1+cos(A-B)]/2
=4+[7cosC+cos(A-B)]/2
=4+[-7cos(A+B)+cos(A-B)]/2
=4+4sinAsinB-3cosAcosB
=4
∴|a|=2