The number, whose square is equal to the difference between

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The number, whose square is equal to the difference between the squares of 975 and 585, is :

1. 780
2. 390
3. 1560
4. 130

Correct Option: A

Let the required number be x.
As per given information,
x² = (975)² – (585)²
⇒ x² = (975 + 585) (975 – 585)
⇒ x² = 1560 × 390
⇒ x = √1560 × 390
= √13 × 12 × 3 × 13 × 10 × 10
= 780