The Super Square-5 Shortcut

Ever wished you could square numbers like $35^2$ or $85^2$ in seconds, all in your head? Get ready to amaze yourself!

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

The Trick

To square any number ending in 5, follow these two simple steps: 1. Take the digit(s) *before* the 5. Let's call this 'n'. 2. Multiply 'n' by ($n+1$). 3. Append '25' to the result from step 2. That's your answer! Why it works: A number ending in 5 can be written as $(10n+5)$. When you square it, you get $(10n+5)^2 = (10n)^2 + 2(10n)(5) + 5^2 = 100n^2 + 100n + 25 = 100n(n+1) + 25$. This means the result is $n(n+1)$ followed by 25.

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