Trigonometric Identities

Let a/2  = B      so a  = 2B

And we have the identity that

cos 2B  = 2cos^2 B - 1        add 1 to both sides

cos 2B + 1  = 2 cos^2 B        divide both sides by 2

cos 2B + 1

________    =  cos^2 B       take both roots

     2

±√ [ ( cos 2B + 1 ) / 2 ]  = cos B

So.....

True  !!!