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: a3 – b3 = 91 and a – b = 1 Then (a – b)3 = (1)3 ⇒ a3 – b3 – 3ab(a – b) = 1 ⇒ 91 – 3ab(1) = 1 ⇒ 3ab = 90 $\therefore$ ab = 30 Hence, the correct answer is 30.
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Question : What is the possible value of (a + b + c) – 3, if a2 + b2 + c2 = 9 and ab + bc + ca = 8?
Option 1: 5
Option 2: 3
Option 4: 2
Question : If (a – b) = 1 and ab = 6, then what is the value of (a3 – b3)?
Option 1: 21
Option 2: 23
Option 3: 19
Option 4: 25
Question : If a + b + c = 1, ab + bc + ca = –22 and abc = –40, then what is the value of a3 + b3 + c3 ?
Option 1: 67
Option 2: –53
Option 3: –51
Option 4: 27
Question : For what value of k will the expression x6 – 18x3 + k be a perfect square?
Option 1: –9
Option 2: –81
Option 4: 81
Question : If (a – b) = 1, what is the value of (a3 – b3)?
Option 1: a2 + ab + b2
Option 2: (a + b)2 + 3ab
Option 3: a2 – ab + b2
Option 4: a2 + 2ab + b2
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