for the cubic: f(x) = ax^3+bx^2+cx+d, a is not zero, find conditions on a,b,c and d to ensure that: a) f is always increasing on (-infinity, +infinity

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neela's profile pic

neela | High School Teacher | (Level 3) Valedictorian

Posted on

f(x) = ax^3+bx^2+cx+d .

If a function is increasing, then its first derivative should be positive.

So f'(x) > 0.

f'(x) = (ax^3+bx^2+cx+d)' > 0.

f'(x)  = 3ax^2+2bx+c > 0.

Therefore  if a is positive an the discriminant (2b)^2 - 4(3a)c < 0,  or 4b^2-12ac < 0 , Or  b^2 - 3ac< 0 then for all x,  f'(x) > 0 and f(x) is an increasing function.

Therefore if a is positive and if b^2-3ac < 0, then for all x f(x) is an increasing function.

 

giorgiana1976's profile pic

giorgiana1976 | College Teacher | (Level 3) Valedictorian

Posted on

For f(x) to be increasing, the derivative of f(x) has to be positive.

We'll determine the first derivative:

f'(x) =  (ax^3+bx^2+cx+d)'

f'(x) = 3ax^2 + 2bx + c

For the expression of the first derivative to be positive, we'll impose the constraint that the discriminant delta to be negative.

delta = (2b)^2 - 4*3a*c

delta = 4b^2 - 12ac

delta < 0

4b^2 - 12ac < 0

We'll divide by 4:

b^2 - 3ac < 0

We'll add 3ac both sides:

b^2 < 3ac

The constraint for f(x) to be increasing over the interval (-infinite, +infinite) is that: b^2 < 3ac.

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omotshow | Student, College Freshman | eNotes Newbie

Posted on

when would it then be decreasing from (-infinity to +infinity)?

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