Question

f(x) = x^4 - 40 x^3

State the values of x for which f is decreasing. Use +infinity or -infinity for the symbols.

and State the values of x for which f is concave down

Answer 1

hers what i have: the values of x when slope = 0 is (0,30)
max. and mim. are: none and 30
one of the inflection points is (0,?)