Sum of first n natural numbers is n(n+1)/2. Prove by induction. (NCERT Class 11 – Important)

Answer: Base n=1: LHS=1, RHS=1(2)/2=1 true. Assume true for n=k: 1+2+…+k=k(k+1)/2. For k+1: sum = k(k+1)/2 + (k+1) = (k+1)(k+2)/2. Hence true for k+1. By induction, formula holds for all n.