MathCastle
Proof WritingHardInduction

Induction · Hard problem

"Claim": n2≥5n−5n^2 \ge 5n - 5 for every positive integer nn. The base n=1n = 1 holds (1≥01 \ge 0), and the step (k+1)2=k2+2k+1≥5k−5+2k+1≥5(k+1)−5(k + 1)^2 = k^2 + 2k + 1 \ge 5k - 5 + 2k + 1 \ge 5(k + 1) - 5 needs 7k−4≥5k7k - 4 \ge 5k, i.e. k≥2k \ge 2. For how many positive integers nn is the claim actually false?