cos(-17π/4)
=cos(-π/4-4π)
=cos(-π/4)
=cos(π/4)
=√2/2
sin(-26π/3)
=sin(-26π/3+8π)
=sin(-2π/3)
=-sin(2π/3)
=-sin(π-π/3)
=-sin(π/3)
=-√3/2