a是第四象限角,cosa=4/5,tana=-3/4
sin(π/4-a)=sinπ/4cosa-cosπ/4sina=7√2/10
cos(π/4+a)=cosπ/4cosa-sinπ/4sina=7√2/10
tan(a-π/4)=(tana-tanπ/4)/(1+tanatanπ/4)=-7