this is a question that was part of an mcq test. and i did'nt have any clue as to how to begin! finally i just tried plugging in values for A,B,C to see if they worked. for ex. i put A=B=C=60 (degrees)- equilateral and so on..but i'd like to know the logical way to do it... if A,B,C are the vertices of a triangle, and if cosBcosC + sinAsinBsinC = 1 is the triangle a) equilateral b) isosceles c) right angled isosceles d) right angled but not isosceles e) none of the above.