黑白大熊猫
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共回答了11个问题采纳率:90.9% 举报
AC 的最短长度为4+2
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
米,此时 BC 的长度为2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
米.
本试题主要考查了解三角形在实际生活中的运用。
设 BC = x 米( x >2), AC = y 米,则 AB = y -1.
在△ ABC 中,由余弦定理,得( y -1)
2 = y
2 + x
2 -2 xy cos60°.
所以 y =
![](https://img.yulucn.com/upload/1/d7/1d73f53cd8055d30300df26178513442_thumb.jpg)
( x >2).……………8分
法一: y =
![](https://img.yulucn.com/upload/1/d7/1d73f53cd8055d30300df26178513442_thumb.jpg)
=( x -2)+
![](https://img.yulucn.com/upload/9/ef/9ef3c21072783dd1b4eae8e3bcf78d68_thumb.jpg)
+4≥4+2
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
.
当且仅当 x -2=
![](https://img.yulucn.com/upload/9/ef/9ef3c21072783dd1b4eae8e3bcf78d68_thumb.jpg)
,即 x =2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
时, y 有最小值4+2
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
.
法二: y ′=
![](https://img.yulucn.com/upload/e/83/e83657d9fc6d7d7cf0f0bd2a41bdfe4f_thumb.jpg)
=
![](https://img.yulucn.com/upload/4/e6/4e64d428290759e1a40d18c55b215d6a_thumb.jpg)
.
由 y ′=0得 x =2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
.因为当2< x <2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
时, y ′<0;当 x >2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
时, y ′>0,
所以当 x =2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
时, y 有最小值4+2
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
.
答: AC 的最短长度为4+2
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
米,此时 BC 的长度为2+
![](https://img.yulucn.com/upload/a/fc/afccc641e37960f41187acff68da3a36_thumb.jpg)
米.……………15分
1年前
1