Squaring Numbers Ending in 5 - Lightning Fast!

Ever wished you could square numbers like $35^2$ or $75^2$ in mere seconds, without a calculator? 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 this be 'N'. 2. Multiply 'N' by 'N + 1'. 3. Append '25' to the result from step 2. Why it works: A number ending in 5 can be written as $ (10N + 5) $. Squaring this gives $ (10N + 5)^2 = (10N)^2 + 2(10N)(5) + 5^2 = 100N^2 + 100N + 25 = 100N(N+1) + 25 $. So, 'N(N+1)' forms the first part, and '25' is appended!

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