Posted by andy on Thursday, July 18, 2013 at 12:40pm.
How many integer values of a are there such that
f(x)=x^+ax^2+8ax+25
has no local extrema?

calculus  Steve, Thursday, July 18, 2013 at 1:10pm
Assuming you meant
x^3+ax^2+8ax+25
we want a derivative with no zeros. So,
f' = 3x^2 + 2ax + 8a
f" = 6x+2a
this will have no zeros if the discriminant is negative, so we need
(2a)^2  4(3)(8a) < 0
4a^2  96a < 0
4a(a24) < 0
So 0<a<24
At a=0 and a=24, f"(0) when f'=0, so there's an inflection point, so no extrema.
So, 0 <= a <= 24
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