Fast Square Fives! 🖐️

Ever wish you could square big numbers ending in 5 in seconds, without a calculator? Unlock the secret to instant squares!

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

The Trick

The trick: To square any number ending in 5 (e.g., $N = 10a+5$), simply identify the digit(s) before the 5. Let's call this 'a'. Then, multiply 'a' by the next consecutive integer, $(a+1)$. Finally, append '25' to the end of this product. That's your answer!\n\n**Why it works:** This comes from the algebraic expansion $(10a+5)^2 = (10a)^2 + 2(10a)(5) + 5^2 = 100a^2 + 100a + 25 = 100a(a+1) + 25$. The $a(a+1)$ part becomes the leading digits, and $25$ is always the suffix.

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