sin（π/4＋a）=1/3 =>sinπ/4·cosa+cosπ/4·sina=1/3=>cosa+sina=(√2)/3∴(cosa+sina)²=1+2sina·cosa=1+sin2a=[(√2)/3]²=2/9∴sin2a=-7/9cos（π/2–2a）=sin2a=-7/9