Q505
If x + y = 3, then what is the value of x3 + y3 + 9xy?
A.
15
B.
81
C.
27
AnswerD.
9
Answer: Option C
Solution
Answer: Option C
Solution:
x + y = 3Put 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