📐 余弦定理计算器Law of Cosines Calculator
用余弦定理 c² = a² + b² − 2ab·cos C 解三角形：已知两边及夹角（SAS）求第三边和其余角，或已知三边（SSS）求三个角，同时给出面积和周长。Solve a triangle with the law of cosines, c² = a² + b² − 2ab·cos C: from two sides and the included angle (SAS) find the third side and the other angles, or from three sides (SSS) find all three angles, plus area and perimeter.
使用说明与常见问题How-to & FAQ
「余弦定理计算器」是什么？What is the Law of Cosines Calculator tool?
用余弦定理 c² = a² + b² − 2ab·cos C 解三角形：已知两边及夹角（SAS）求第三边和其余角，或已知三边（SSS）求三个角，同时给出面积和周长。Solve a triangle with the law of cosines, c² = a² + b² − 2ab·cos C: from two sides and the included angle (SAS) find the third side and the other angles, or from three sides (SSS) find all three angles, plus area and perimeter.
如何使用「余弦定理计算器」？How do I use the Law of Cosines Calculator tool?
在「余弦定理计算器」中，填写「已知」、「边 a」、「边 b」、「夹角 C（度）」、「边 c」等参数，结果会随输入实时计算并显示在下方。In the Law of Cosines Calculator: Fill in "Given", "Side a", "Side b", "Included angle C (degrees)", "Side c"; results are recalculated instantly as you type.
余弦定理是什么？What is the law of cosines?
c² = a² + b² − 2ab·cos C，其中 C 是边 c 的对角。C = 90° 时就是勾股定理。c² = a² + b² − 2ab·cos C, where C is the angle opposite side c. When C = 90° it reduces to Pythagoras.
什么时候用余弦定理？When do I use it?
已知两边及其夹角（SAS）或三边（SSS）时用余弦定理；已知两角一边或两边及一边对角时用正弦定理。Use it for SAS (two sides and the included angle) or SSS (three sides); for ASA, AAS or SSA use the law of sines.
计算精度如何？How precise are the results?
「余弦定理计算器」使用 JavaScript 双精度浮点数（约 15–16 位有效数字）；极大或极小的数值可能存在精度误差。The Law of Cosines Calculator uses double-precision floating point (about 15–16 significant digits); very large or very small values may lose precision.
「余弦定理计算器」免费吗？需要注册吗？Is the Law of Cosines Calculator free? Do I need an account?
「余弦定理计算器」完全免费，无需注册或安装，打开网页即可使用，手机和电脑都能用；输入的内容只在你的浏览器本地处理。The Law of Cosines Calculator is completely free with no sign-up or install and works on phones and desktops; what you enter is processed only in your browser.