Question

please help to find cubic equation of f' (x) =0 at(-1;-3,5)
f'(x)<0 for x<-1 and x>2
f'(x)>0 for -1 15 years ago

Answer 1

You mean f' (the derivative of f is cubic) and want to evaluate f' at -1,-3 and 5?

Answer 2

If so, then take the genral biquad i.e. f(x)=ax^4+bx^3+cx^2+dx+e.
Use f(0)=0,f(-2)=0,f(4)=0 and get e=0 and 2 equations with 4 unknowns.
Find derivative of f and use the inequalities given.