sin[(4n-1/4)π-α]+cos[(4n-1/4)π-α]
=sin(4nπ-π/4-α)+cos(4nπ-π/4-α)
=sin(-π/4-α)+cos(-π/4-α)
=-sin(π/4+α)+cos(π/4+α)
=-sinπ/4cosα-cosπ/4sinα+cosπ/4cosα-sinπ/4sinα
=√2/2(-cosα-sinα+cosα-sinα)
=-√2sinα