Multiple choice

$(\text{What approximate value should come in place of question mark (?) in the following questions?})$ $((42.05)^2 - (28.9)^2 - (21.9)^2 =?)$

  1. 470

  2. 439

  3. 420

  4. 680

  5. 760
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

This uses the difference of squares: a² - b² = (a-b)(a+b). First compute (42.05)² - (28.9)² = (42.05-28.9)(42.05+28.9) = 13.15 × 70.95 ≈ 933. Then subtract (21.9)²: 933 - 479.61 ≈ 453.39 ≈ 439 (nearest option). Alternatively, calculate squares directly: 1767.20 - 835.21 - 479.61 = 452.38 ≈ 439.

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