# What is the Value of Cos 30 Degrees ?

## Solution :

The value of cos 30 degrees is $$\sqrt{3}\over 2$$.

Proof :

Consider an equilateral triangle ABC with each side of length of 2a. Each angle of $$\Delta$$ ABC is of 60 degrees. Let AD be the perpendicular from A on BC.

$$\therefore$$   AD is the bisector of $$\angle$$ A and D is the mid-point of BC. $$\therefore$$   BD = DC = a and $$\angle$$ BAD = 30 degrees.

In $$\Delta$$ ADB, $$\angle$$ D is a right angle, AB = 2a and BD = a

By Pythagoras theorem,

$$AB^2$$ = $$AD^2$$ + $$BD^2$$  $$\implies$$  $$2a^2$$ = $$AD^2$$ + $$a^2$$

$$\implies$$  $$AD^2$$ = $$4a^2$$ – $$a^2$$ = $$3a^2$$   $$\implies$$  AD = $$\sqrt{3}a$$

Now, In triangle ADB, $$\angle$$ BAD = 30 degrees

By using trigonometric formulas,

$$cos 30^{\circ}$$ = $$base\over hypotenuse$$ = $$b\over h$$

$$cos 30^{\circ}$$ = side adjacent to 30 degrees/hypotenuse = $$AD\over AB$$ = $$\sqrt{3}\over 2a$$ = $$\sqrt{3}\over 2$$

Hence, the value of $$cos 30^{\circ}$$ = $$\sqrt{3}\over 2$$