Here's a tip :

Should you forget the special angles during test, use calculator to help you recall.

Square the value to see what fraction you get.

For example ,

keying sin60° into calculator gives you 0.866025404. just from looking, you can't really tell what its exact form is.

Immediately square this value by pressing the x² function on your calculator.

You would see 0.75 or ¾

Now since the square of sin60° is ¾, and sin60° is positive,

then sin60° is just √(¾) = √3/2


It's also useful for other angles

Eg. sin 225° gives you -0.707106781

Squaring immediately gives 0.5 or ½

Since the square of sin225° is ½, and
sin225° itself is negative,

Then sin225° = -√(½) = -1/√2

Another way is to extract 30 and 60 degrees (you know how to convert radians to degrees right? if the angle is given in radians, you can convert to degrees) from an equilateral triangle of side 2 cm and 45 degrees from an isosceles right angled triangle of side 1 cm.