If a – b = 1 and a3 – b3 = 61, then the value of ab

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If a – b = 1 and a³ – b³ = 61, then the value of ab will be

1. –20
2. 20
3. 30
4. 60

Correct Option: B

a³ − b³
= (a – b)³ + 3ab (a – b)
⇒ 61 = 1 + 3ab × 1
⇒ 61 – 1 = 3ab = 60

⇒ ab =	60	= 20
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