Let S=\x [- , ] : x( x+ x)=a, a Z\. Then n(S) is equal to :
- 3
- 6
- 7
- 9
Answer
(4) 9
Solution
x( x+ x)= 2x+ x x= 1- 2x2+ 2x2
= 12+ 2x- 2x2 [ 1- 22, 1+ 22 ] [-0.21, 1.21]
∴ a Z a=0 or a=1
a=0: x( x+ x)=0
x=0: x=- ,0, ; or x=-1: x=- 4, 3 4 → 5 values
a=1: 2x+ x x= 2x+ 2x
x( x- x)=0
x=0: x= 2; or x=1: x= 4,- 3 4 → 4 values
All 9 values are different ∴ n(S)=5+4=9
Answer: (4)