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If (a2 + 1/a2) = 17/4, then what is (a3 - 1/a3) equal to?

Correct Answer: 63/8

Explanation:

(a2 + 1/a2) = 17/4
subtracts 2 and Add 2 from above equation, we will get
⇒ a2 + 1/a2 - 2 + 2 = 17/4
⇒ a2 + 1/a2 - 2a x 1/a + 2 = 17/4
Now apply the formula, x2 + x2 - 2xy
⇒ (a - 1/a)2 + 2 = 17/4
⇒ (a - 1/a)2 = 17/4 - 2
⇒ (a - 1/a)2 = (17 - 8)/4
⇒ (a - 1/a)2 = 9/4
⇒ (a - 1/a) = √9/4
(a -1/a) = 3/2
After that, cubing On both sides, we get
(a -1/a)3 = (3/2)3
Apply the formula (x – y)3 = x3 – 3x2y + 3xy2 – c3
(x – y)3 = x3 – 3xy(x - y) – y3
⇒ a3 - 1/a3 - 3 x a x 1/a (a - 1/a) = 27/8
⇒ a3 - 1/a3 = 27/8 + 3 x (3/2)
⇒ a3 - 1/a3 = 27/8 + 9/2
⇒ (a3 - 1/a3) = 63/8


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