Question

If x+y =3 and xy = -18 find value of xcube +y cube

Answer 1

Step-by-step explanation:

${x}^{3} + {y}^{3} = {(x + y)}^{3} - 3xy(x + y) \\ = {(3)}^{3} - 3 \times ( - 18) \times 3 \\= 27 + 162 \\ = 189$

Answer 2

x + y = 3

xy = -18

______________[GIVEN]

x³ + y³ = ?

x + y = 3

• Take cube on both sides

=> (x + y)³ = (3)³

=> x³ + 3xy (x + y) + y³ = 27

=> x³ + y³ + 3(-18) (3) = 27

=> x³ + y³ - 162 = 27

=> x³ + y³ = 27 + 162

______________________________

x³ + y³ = 189

______________[ANSWER]

☞ Used identify :

(a + b)³ = a³ + 3ab (a + b) + b³

_______________________________