Question

What is cos 30?

Answer 1

go here to:
http://www.mathsisfun.com/sine-cosine-tangent.html
http://www.rapidtables.com/calc/math/Cos_Calculator.htm

cos 0.8660254

Answer 2

It depends what units you are working in:

cos(30 degrees) = 0.86602
cos(30 radians) = 0.15425

Answer 3

0.15425144988

Answer 4

I got disconnect.

All of those answers are not good.

\[\cos(A-B)=\cos A \cos B + \sin A \sin B\]\[\cos(30)=\cos(45-15)=\cos45 \cos 15 + \sin 45 \sin 15\]

Both sin(45) and cos(45) are \[\cos(45)=\color{red}{ \frac{\sqrt{2}}{2} }=\sin(45)\] (because in a 45-45-90 triangle all legs are the same and hypotenuse is times sqrt2 greater then each leg.)

So,

Answer 5

\[\sin(15)=\color{red}{ \frac{\sqrt{6}-\sqrt{2}}{4} }\]\[\cos(15)=\color{red}{ \frac{\sqrt{6}+\sqrt{2}}{4} }\]

Answer 6

plug in the values
\[\cos(45-15)= \color{red}{ \frac{\sqrt{2}}{2} \times }\color{red}{ \frac{\sqrt{6}+\sqrt{2}}{4} }~~+~~ \color{red}{ \frac{\sqrt{2}}{2} \times }\color{red}{ \frac{\sqrt{6}-\sqrt{2}}{4} }\]

Answer 7

\[\cos(45-15)=\color{blue}{ \frac{\sqrt{2}~(\sqrt{6}+\sqrt{2})}{2 \times 4} +\frac{\sqrt{2}~(\sqrt{6}-\sqrt{2})}{2 \times 4} }\]\[\cos(45-15)=\color{blue}{ \frac{\sqrt{12}+\sqrt{4}}{8}}+\frac{\sqrt{12}-\sqrt{4}}{8}=\frac{\sqrt{12}}{8}=\frac{\sqrt{3}}{4}\]