cosa=-3/5,且a∈(π,3π/2),
sina=-4/5
tana=4/3
tan(a-π/4)
=(tana-tanπ/4)/(1+tanatanπ/4)
=(tana-1)/(1+tana)
=(4/3-1)/(1+4/3)
=(1/3)/(7/3)
=1/7