问题
填空题
已知AD是△ABC的中线,若∠A=120°,
|
答案
∵
•AB
=-2=|AC
||AB
|cosA,∠A=120°,----------------------(7分)AC
∴|
||AB
|=4----------------------------------(8分)AC
∵|
|=AD
( 1 2
+AB
),AC
∴|
|2=AD
(|1 4
|2+|AB
|2+2 AC
•AB
)=AC
(|1 4
|2+|AB
|2-4)AC
≥
(2|1 4
||AB
|-4)=1---------------(10分)AC
∴|
|min=1----------------------------------(12分)AD
故答案为:1.