Question Details

For real values of x, the range of the function f(x) = 2x3 2 x2 + 4x 6    is

Options

A

( , 18 ] [ 1 , )

B

( , 14 ] [ 1 , )

C

( , 18 ] [ 12 , )

D

( , 14 ] [ 12 , )

Show Answer

Correct Answer :

Option C

( , 18 ] [ 12 , )

Solution :

The correct answer is (-,18][12,).

To find the range of the function for real values of x, let y=f(x):
y=2x-32x2+4x-6

Cross-multiply to clear the fraction and rearrange the equation into a quadratic form in terms of x:
y(2x2+4x-6)=2x-3
2yx2+4yx-6y=2x-3
2yx2+(4y-2)x+(3-6y)=0

Since x is a real number, the quadratic equation must have real roots. This requires the discriminant (D) to be greater than or equal to zero (D0):
D=b2-4ac0
(4y-2)2-4(2y)(3-6y)0

Expand and simplify the algebraic terms:
(16y2-16y+4)-8y(3-6y)0
16y2-16y+4-24y+48y20
64y2-40y+40

Divide the entire inequality by 4:
16y2-10y+10
Factor the quadratic inequality:
(8y-1)(2y-1)0

Solving (8y-1)(2y-1)0 yields:
y18 or y12

Expressed in interval notation, the range of the function is:
(-,18][12,)

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