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If 3a – 2b = 11 and ab = 12, then find the value of 27a3 – 8b3. |
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Answer» We know that (a – b)3 = a3 – b3 – 3ab(a – b) Using this identity, (3a – 2b)3 = (3a)3 – (2b)3 -3 x 3a x 2b (3a – 2b) ⇒ (3a – 2b)3 = 27a3 – 8b3 – 18ab(3a – 2b) Now substituting 3a – 2b = 11 and ab = 12, we get (11)2 = 27a3 – 8b3 – 18 x 12 x 11 ⇒ 1331 = 27a3 – 8b3 – 2376 ⇒ 27a3 – 8b2 = 1331 + 2376 ⇒ 27a3 – 8b3 = 3707 |
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