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When two numbers are the same distance either side of a round number, the answer is that number squared minus the distance squared. 19 × 21 is one less than 20 × 20.
✓ You passed this lesson’s test.
Middle square — “(a − d) × (a + d) = a² − d²”
Use it for: Pairs like 29 × 31, 47 × 53, 97 × 103 — the pattern 3 × 5 = 15 = 4 × 4 − 1 works for every square.
Check: 48 × 52 = 48 × 50 + 48 × 2 = 2400 + 96 = 2496 ✓
Work it out in your head or on paper, type the answer and press Check. Stuck? Press “Show me how”.
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20 questions. Check each answer, then go to the next one. Get 16 or more right to pass.
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