Given function f(x)= x^3-4x+5x in an agricultural experiment result. Find the open interval where f is concave down i.e where the second derivative is less than zero.
assuming you meant f(x) = x^3 - 4x^2 + 5x
so what's the problem? f'' is very easy to find:
f' = 3x^2-8x+5
f'' = 6x-8
so, where is 6x-8 < 0 ??