I'm not sure I understand what you mean by 'defination'...

You might want to use the compound angles formula?

\tan (A+ B) = \frac{\tan A + \tan B}{1 - \tan A\tan B}

Put A = 180, B = 45

I think what the OP meant is to use the definition:


tan \theta = \frac {sin \theta} {cos \theta}


Finding the sine and cosine of 225 degrees is pretty straight forward - just remember that it's 45 degrees "past" 180, which puts it in the 3rd quadrant. Varya - can you take it from here?