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If a + b = 7 and a^3 + b^3 = 44, what is the value of the sum 1/a + 1/b? Express your answer as a common fraction.

 Oct 22, 2022
 #1
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 a + b = 7,   

a^3 + b^3 = 44, solve for a, b

 

a≈3.5 - 1.41 i  and   b≈3.5 + 1.41 i

a≈3.5 + 1.41 i ∧ b≈3.5 - 1.41 i

 

1 / [3.5 + 1.41 i]  +  1 / [3.5 - 1.41 i] =70,000 / 142,381 = 0.4916386315589861.......etc.

 Oct 22, 2022
 #2
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\({1 \over a} + {1 \over b} = {b \over ab} + {a \over ab} = {a + b \over ab} = {7 \over ab} \)

\((a+b)^3 = a^3 + b^3 + 3a^2b+3ab^2 = 343\)

\(44 + 3a^2b+3ab^2 = 343\)

\(3a^2b+3ab^2 = 299\)

\(3ab(a+b) = 299\)

\(3ab \times 7 = 299\)

\(21ab = 299\)

\(ab = {299 \over 21}\)

\({7 \over {299 \over 21}} = {{147 \over 7} \over {299 \over 21}} = \color{brown}\boxed{147 \over 299}\)

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 Oct 22, 2022

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