已知tana=—1/3,cosb=根号5/5,其中a,b属于(0,兀)(1)求cosa的值;(2)求sin(a+b)的值

tana=—1/3 a∈(π/2,π)
sina/cosa=-1/3,
sin&2 a/cos&2 a=1/9
cos&2 a=9sin&2 a
sin^2 a+cos^2 a=1
10sin&2 a=1
a,b€(0,兀)
sina=根号10/10
cosa=-3根10/10
cosb=根号5/5,
sinb=根号（1-1/5）=根号4/5=2根号5/5
sin（a+b)=sinacosb+cosasinb
=(根10/10)*根5/5-(3根10/10)*2根号5/5
=根50/50-6根50/50
=-5根50/50
=-根2/2