cos²¼π+sin²四分之十七-sin﹣三分之八π*cos-六分之三十一π-tan¾π+cos(-1260)
=cos3/4π+sin²1/4π+sin2/3π* cos5/6π +tan1/4π+cos180
=-(√2/2)+(√2/2)²+(√3/2)* (-√3/2) +1-1
=-(√2/2)+1/2-3/4
=-(2√2+1)/4