Let f: R R be defined as f(x)= 2x2-3x+23x2+x+3. Then f is :
- both one-one and onto
- one-one but not onto
- onto but not one-one
- neither one-one nor onto
Answer
(4) neither one-one nor onto
Solution
Not one-one: put f(x)=1
2x2-3x+2=3x2+x+3
x2+4x+1=0 x=-2 3
Two different values of x have the same image 1, so f is not one-one.
Not onto: let y=f(x)
(3y-2)x2+(y+3)x+(3y-2)=0
x is real, the discriminant is non-negative (for y= 23 we get x=0, which also satisfies the result below)
(y+3)2-4(3y-2)2 0
(7-5y)(7y-1) 0
17 y 75
Range of f= [ 17, 75 ] , so f is not onto.
Hence f is neither one-one nor onto.
Answer: (4)