cosA=4/5求Sin^2(B+C)/2
cosA=-cos(B+C)=4/5
Sin^2(B+C)/2=[1-cos(B+C)]/2
=[1+4/5]/2=9/10
s=3 ,b=2
s=cbsinA/2 ,c=5 ,(sinA=3/5)
a^2=b^2+c^2-2bccosA
=4+25-20*4/5=13
a=√13