tana=-5/12,tanπ/4=1
1,tan(a-π/4)=(tana-tanπ/4)/(1+tana*tanπ/4)=(-5/12-1)/(1-5/12)=-17/7
2,万能公式：sin2a=2tana/(1+tan²a)=(-5/6)/(1+25/144)=-120/169
cos2a=(1-tan²a)/(1+tan²a)=(1-25/144)/(1+25/144)=119/169
带入1/2(sin2a)+cos2a,即可
(所求的这个式子有歧义,楼主自己带入喽)