Solving the Mystery of an Unfamiliar Trig Identity

[thenewbosco]

Today the professor wrote down the following for use in an integral:

$$\frac{1}{2}cos(2\theta)=sin\theta cos\theta$$

i am not sure is this is correct. i have tried to prove this and cannot, i have also not found it in an table of trig identities. Is this a valid identity? i am not sure since this prof always makes mistakes

 

[courtrigrad]

It should be $$\frac{1}{2}\sin 2\theta = \sin\theta \cos\theta$$

 

[dextercioby]

He probably meant

$$\int \cos 2\theta = \sin\theta \cos\theta +c$$

Daniel.