Question : a3 – b3 = 91 and a – b = 1, what is the value of (ab)?
Option 1: 27
Option 2: 6
Option 3: 9
Option 4: 30
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Correct Answer: 30
Solution : Given: a 3 – b 3 = 91 and a – b = 1 Then (a – b) 3 = (1) 3 ⇒ a 3 – b 3 – 3ab(a – b) = 1 ⇒ 91 – 3ab(1) = 1 ⇒ 3ab = 90 $\therefore$ ab = 30 Hence, the correct answer is 30.
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