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JAMB 1987 · UME · Q24

An (n−2)2(n - 2)^2-sided figure has nn diagonals (according to this rule). Find the number nn of diagonals for a 25-sided figure.

Worked solution (try it first)
  1. By the rule, a 25-sided figure has (n−2)2=25(n - 2)^2 = 25.
  2. Take the square root: n−2=5n - 2 = 5 (the number is positive).
  3. Add 2: n=7n = 7, option A.

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