IF you are asked this ques. in exam first of all substituting specific values of a,b,c like 0 1 etc. cannot be accepted.Since that doesn't prove anything,I mean there are a whole lot of values which satisfy a+b+c=180,you cant possibly substitute all the infinite values and then evaluate them or something.That's why these are not asked as objective questions.Even though all of us know this we tend to subsitute values and find the answer to certain questions in mock tests,etc..One has to realie that although his may increase ur marks in that particular test,It does not improve your logical capabilities or other skill sets,I mean why do all this crap and cheating even while we r preparing,