Que: Prove that sin^4 A - sin^2 A cos^2 A +cos^4 A + 3 sin^2 A cos^2 A - (sin^2 A + cos^2 A) + sin^3 A cosec^2 A = sin A. Student: Multiply R.H.S. & L.H.S. by 0 i.e. 0=0. Teacher gave marks: 10/10 * 0 = 0. For you: Can you find out the mathematical flaw in the answer given by the student and the marks given by the teacher which have given us misleading results? NOTE: We often multiply both sides by a rational no. in such questions but if we do that with 0 it becomes misleading. What's the reason?