找出 sin 135° 的准确值

三角学是一门数学学科，研究直角三角形的边长和角之间的关系。三角函数，也称为测角函数、角函数或圆函数，是建立角度与直角三角形两条边之比之间关系的函数。六个主要的三角函数是正弦、余弦、正切、余切、正割和余割。

Angles defined by the ratios of trigonometric functions are known as trigonometry angles. Trigonometric angles represent trigonometric functions. The value of the angle can be anywhere between 0-360°.

如上图中的直角三角形所示：

• 斜边：与直角相对的边是斜边，它是直角三角形中最长的边，与90°角相对。
• 底：角 C 所在的一侧称为底。
• 垂直：考虑角度 C 的对边。

三角函数

三角函数有 6 个基本的三角函数，它们是正弦、余弦、正切、余割、正割和余切。现在让我们看看三角函数。六个三角函数如下，

sine: It is defined as the ratio of perpendicular and hypotenuse and It is represented as sin θ

cosine: It is defined as the ratio of base and hypotenuse and it is represented as cos θ

tangent: It is defined as the ratio of sine and cosine of an angle. Thus the definition of tangent comes out to be the ratio of perpendicular and base and is represented as tan θ

cosecant: It is the reciprocal of sin θ and is represented as cosec θ.

secant: It is the reciprocal of cos θ and is represented as sec θ.

cotangent: It is the reciprocal of tan θ and is represented as cot θ.

补角和补角的三角恒等式

• 互补角：和等于90°的一对角
• 补角：和等于 180° 的一对角

互补角的恒等式是

sin (90° – θ) = cos θ

cos (90° – θ) = sin θ

tan (90° – θ) = cot θ

cot (90° – θ) = tan θ

sec (90° – θ) = cosec θ

cosec (90° – θ) = sec θ

补角的恒等式

sin (180° – θ)  = sin θ

cos (180° – θ) = – cos θ

tan (180° – θ) =  – tan θ

cot  (180° – θ) = – cot θ

sec (180° – θ) = – sec θ

cosec (180° – θ) = – cosec θ

找出 sin 135° 的准确值。

解决方案：

Since, we know that sin is positive in the 1st and 2nd Quadrant,

here, 135° lies in the 2nd Quadrant, then

By the Trigonometric Identity of Supplementary Angles,

We know that sin (180° – θ)  = sin θ

Hence,

sin 135° = sin(180° – 45°)

= sin 45°                 {As given by Identity}

= 1/√2

类似问题

问题1：sin 150°的准确值是多少？

解决方案：

Here sin is positive only in the 1st and 2nd Quadrant.

150° lies in the 2nd Quadrant.

Therefore     

sin (180° – θ) = sin θ

sin (150°) = sin (180° – 30°)

sin (150°) = sin (30°)                  

sin (150°) = 1/2       

So the exact value of sin 150° is 1/2

问题2：cos 150°的准确值是多少？

解决方案：

Here cos is positive only in 1st and 4th Quadrant.

150° lies in 2nd Quadrant.

Therefore cos(180° – θ) = – cos θ

cos(150°) =  cos(180° – 30°)

 cos(150°) = -cos(30°)                  

cos (150°) = -√3/2      { as per the trigonometry value table }  

So the exact value of cos 150° is -√3/2