∵
a +
b =
c
∴ (
a +
b ) 2 =
c 2
即
a 2 +
b 2 +2
a •
b =
c 2
∵|
a |=|
b |=|
c |
∴
a •
b =-
1
2
b 2
代入向量的夹角公式可得 cos<
a ,
b > =
a •
b
|
a ||
b | =
-
1
2
b 2
b 2 = -
1
2
∴ <
a ,
b >=120°
故答案为:120°
∵
a +
b =
c
∴ (
a +
b ) 2 =
c 2
即
a 2 +
b 2 +2
a •
b =
c 2
∵|
a |=|
b |=|
c |
∴
a •
b =-
1
2
b 2
代入向量的夹角公式可得 cos<
a ,
b > =
a •
b
|
a ||
b | =
-
1
2
b 2
b 2 = -
1
2
∴ <
a ,
b >=120°
故答案为:120°