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If x + y = 3 then what is the value...

This question belongs to Arithmetic Ability Algebra
Algebra
?

If x + y = 3, then what is the value of x3 + y3 + 9xy?

Answer & Solution
Correct Answer: Option C
x + y = 3
Put x = 1, y = 2
x3 + y3 + 9xy
= (1)3 + (2)3 + 9 × 2 × 1
= 1 + 8 + 18
= 27

Alternate:
x + y = 3 . . . . . . (1)
On taking cube on both sides
(x + y)3 = (3)3
x3 + y3 + 3xy(x + y) = 27
x3 + y3 + 3xy(3) = 27
x3 + y3 + 9xy = 27
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