cos60 and cos180
how we find these cos,sin like in an example it is given:
cos60= 1/2
cos120=-1/2
how it it just happens .
For cos 60 = 1/2:
Let us assume AB= BC = CA = 2a. Therefore, BD = ½ BC = a.
Now, we know that cos angle = side adjacent to angle/hypotenuse
Therefore, in traingle ABD, cos 60 = BD/AB = a/2a = 1/2
For cos 120 = -1/2:
cos 120 = cos(180-60)
= -cos(60) (Since, cos(180-thita)= -cos(thita))
= -1/2