题目内容：

在三角形ABC中,D为BC的中点,已知AB=向量a,AC=向量b,证向量AD等于a+b/|a+b|

优质解答

AB = a = OB -OA
AC = b = OC -OA
2OD = OB + OC (D为BC的中点)
AD = OD - OA
= (OB+OC)/2 -OA
= [(OB-OA) + (OC-OA)]/2
= (a+b)/2