Extended Principle of Mathematical Induction The Extended Principle of Mathematical Induction states that if Conditions I and II hold, that is,
(I) A statement is true for a natural number $j$.
(II) If the statement is true for some natural number $k \geq j$, then it is also true for the next natural number $k+1$. then the statement is true for all natural numbers $\geq j$. Use the Extended Principle of Mathematical Induction to show that the number of diagonals in a convex polygon of $n$ sides is $\frac{1}{2} n(n-3)$.