∵已知sinx=4/5,且x∈（π/2,π）
∴cosx=-3/5
cos2x=2cos²x-1
=2·9/25-1
=-7/25
sin2x=2sinxcosx
=2·4/5(-3/5)
=-24/25
cos(2x-60º)
=cos2xcos60º+sin2xsin60º
=-7/25·1/2-24/25·√3/2
=-7/50-12√3/25