cos2x=1-2sin^2x=7/8
∵x∈(π/2,π)
∴sinx=1/4
cosx=-√15/4
sin(x+π/6)-sin2x
=sinxcosπ/6+cosxsinπ/6-2sinxcosx
=1/4*√3/2-√15/4*1/4-2*1/4*(-√15/4)
=√3/8
cos2x=1-2sin^2x=7/8
∵x∈(π/2,π)
∴sinx=1/4
cosx=-√15/4
sin(x+π/6)-sin2x
=sinxcosπ/6+cosxsinπ/6-2sinxcosx
=1/4*√3/2-√15/4*1/4-2*1/4*(-√15/4)
=√3/8