April Tip of the Month - Squaring 10 × n + 5:

Apr 30, 2024 | Dundas

Here's a quick tip for squaring a two digit number ending in five:
(10n + 5)² = 100 × n × (n+1) + 25

Hence:
15² = 100 × 1 × 2 + 25 = 200 + 25 = 225
25² = 100 × 2 × 3 + 25 = 600 + 25 = 625
35² = 100 × 3 × 4 + 25 = 1200 + 25 = 1225
45² = 100 × 4 × 5 + 25 = 2000 + 25 = 2025
55² = 100 × 5 × 6 + 25 = 3000 + 25 = 3025
:
95² = 100 × 9 ×10 + 25 = 9000 + 25 = 9025

The trick works for numbers of more than two digits, but becomes less handy for mental math.

April Tip of the Month - Squaring 10 × n + 5:

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