sin(π/4-a)=5/13,0＜a＜π/4a,得sin(a-π/4）=-5/13
得cos(a-π/4)=√（1-sin²(π/4-a)=12/13
sina=sin(a-π/4+π/4）=-5/13×√2/2+12/13×√2/2=7√2/26
cosa=cos(a-π/4+π/4）=12/13×√2/2-（-5/13）×√2/2=17√2/26
3sin²-4sinacosa+cos²a
=(sina-cosa)²+2sin²a-2sinacosa
=(sina-cosa)²+2sina(sina-cosa)
=50/169+2×7√2/26（-5√2/13）
=-20/169