Question

if a-b=-7and ab=2 find a³-b³​

Answer 1

a - b = - 7

ab = 2

__________ [GIVEN]

• We have to find the value of a³ - b³

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a - b = - 7

• Take cube on both sides..

=> (a - b)³ = (-7)³

(a - b)³ = a³ - b³ - 3ab (a - b)

=> a³ - b³ - 3ab (a - b) = - 343

=> a³ - b³ - 3(2) (-7) = - 343

=> a³ - b³ + 42 = - 343

=> a³ - b³ = - 343 - 42

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a³ - b³ = - 385

_____________ [ANSWER]

Answer 2

(a - b)³ = (-7)³

a³ - b³ - 3ab(a - b) = - 343

a³ - b³ - 3×-7×2 = -343

a³ - b³ + 42 = - 343

$\huge{ \bold {a}^{3} \: - \: {b}^{3} \: = \: - 385}$