直角三角形中30度角所对的直角边是斜边的一半

2024-11-02 23:32:37
推荐回答(5个)
回答1:

证法1:

Rt三角形ABC,角C=90°,AB=2BC

延长这条直角边BC至D,使得BD=AB,连接AD

角BCA=角DCA,BD=AB,AC=AC

所以三角形ABC全等于三角形ADC

所以AB=AD,又BD=AB

所以三角形ABD是等边三角形

所以角B=60°

而角BAC=30°

 

证法2:

在三角形ABC的斜边上取点D,使得角CBD=30度

又角B=90度,所以角ABD=60度

因为角A=角ABD=60度,所以三角形ABD为等边三角形

所以AB=AD

又因为角C=角CBD=30度

所以三角形BCD为等腰三角形

BD=CD

所以AD=BD=CD=AB →AC=2AB

证毕

 

证法3; 

利用正弦定理可证如下

   a/sin30=c/sin90

     得c=2a,

其中,a为30度角对的边,c为90度角对的边 

 

证法4:

 

在三角形ACB中,角C等于90度,角A等于30度

作AD平行且等于CB, DB平行且等于AC 。

所以四边形ACBD为矩形。

所以AB=CD 且互相平分

所以CE=1/2AB

回答2:

回答3:

你好,这是直角三角形中,30度角的特点。需要干什么,是证明吗,如果是那就,在直角内做一个30度的角,那30度的两个角内就成为一个等腰三角形,另一个三角形就是等边三角形,就可以了,

回答4:

对的,可以通过sin30=b/c=1/2来验证,b是30度所对的直角边,c是斜边!

回答5:

特殊直角三角形的性质,如果一个直角三角形有一个内角为30°,那么这个30°角所对的直角边等于斜边的一半。

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