The '5-Ender' Square Shortcut! ⚡

Ever wish you could square big numbers like 35, 85, or even 125 in seconds, without a calculator? Unlock a simple trick to do it mentally!

Subject: mental-calculation • Classes: 6–12 • Difficulty: intermediate

The Trick

This trick works for any number ending in 5. Take the digit(s) *before* the 5. Let this be $n$. Multiply $n$ by $(n+1)$. Then, simply append '25' to the result of this multiplication! The 'why' behind it: A number ending in 5 can be written as $(10n+5)$. Squaring it gives $(10n+5)^2 = 100n^2 + 100n + 25 = 100n(n+1) + 25$. This clearly shows $n(n+1)$ forms the prefix and 25 is the suffix.

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