在三角形ABC,角A、B、C所对的边分别为a、b、c且满足cosA/2=2√5/5,向量AB*向量AC=

cosA=2[cos（A/2）]^2-1=3/5
SINA=4/5
AB向量 乘以 AC向量 = AB X AC X COSA =3
AB X AC = 5
面积 = 1/2 乘以 AB X AC 乘以SINA = 2
a^2 = b^2 + c^2 - 2bc cosA =(b+c)^2 - 2bc -2bc cosA
=36 - 10 -10x3/5 =20
a = 2倍根5