Practicing Success
Let $f(x)=x^3+a x^2+b x+5 \sin ^2 x$ be an increasing function on the set R. Then, a and b satisfy |
$a^2-3 b-15>0$ $a^2-3 b+15>0$ $a^2-3 b+15<0$ $a>0$ and $b>0$ |
$a^2-3 b+15<0$ |
$f(x)=x^3+a x^2+b x+5 \sin ^2 x$ is increasing on R $\Rightarrow f'(x)>0$ for all $x \in R$ $\Rightarrow 3 x^2+2 a x+b+5 \sin 2 x>0$ for all $x \in R$ $\Rightarrow 3 x^2+2 a x+(b-5)>0$ for all $x \in R$ $\Rightarrow (2 a)^2-4 \times 3 \times(b-5)<0 \Rightarrow a^2-3 b+15<0$ |