|AB+AC| = |AB-AC|
=> (AB+AC).(AB+AC)=(AB-AC).(AB-AC)
|AB|^2+|AC|^2 + 2AB.AC = |AB|^2+|AC|^2 - 2AB.AC
AB.AC =0
|AB||AC|cosA =0
cosA =0
=>A是直角
|AB+AC| = |AB-AC|
=> (AB+AC).(AB+AC)=(AB-AC).(AB-AC)
|AB|^2+|AC|^2 + 2AB.AC = |AB|^2+|AC|^2 - 2AB.AC
AB.AC =0
|AB||AC|cosA =0
cosA =0
=>A是直角