If $f(x)=2 x^3+9 x^2+\lambda x+20$ is a decreasing function of $x$ in the interval $(-2,-1)$, these

Question

If $f(x)=2 x^3+9 x^2+\lambda x+20$ is a decreasing function of $x$ in the interval $(-2,-1)$, these $A$ is equal to

Professor PC Thomas Classes · Answer

Derivative $f'(x) \le 0$.