If a + b + c = 6 a3 + b3...

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If a + b + c = 6, a³ + b³ + c³ - 3abc = 342, then what is the value of ab + bc + ca?

A. 5

B. 8

C. -7

D. -5

Answer: Option C

Let c = 0
a + b = 6
a³ + b³ = 342
ab = ?
(a³ + b³) = (a + b)[(a + b)² - 3ab]
342 = 6[6² - 3ab]
57 = 36 - 3ab
3ab = -21
ab = -7

This Question Belongs to Arithmetic Ability >> Algebra

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