已知a是第一象限的角,且cosa=5/13,求sin(a+pia/4)/cos(2a+pia/4)的值

sina=12/13
sin(a+45)=sinacos45+sin45cosa=17√2/26
cos(a+45)=cosacos45-sinasin45=-7√2/26
cos(2a+pia/4)
=cos[a+(a+pia/4)]
=cosacos(a+45)-sinasin(a+45)
=-239√2/338
sin(a+pia/4)/cos(2a+pia/4)
=(17√2/26)/(-239√2/338)
=-221/239