Q:

Where am i going wrong in solving this equation? 

Fing the least value of $a$ for which $f(x)$ is increasing, where
  $$f(x)=2e^x-ae^{-x}+(2a+1)x-3$$

What i tried
for increasing $f'(x)\ge 0, \forall x\in \mathbb R$. So $$f'(x)=2e^x+ae^{-x}+2a+1\geq0$$ and multiplying through by $e^x$ we  get$$f'(x)=2e^{2x}+a+(2a+1)e^x\ge0$$and now put $y=e^x$
$$\implies2y^2+(2a+1)y+a\ge0$$ So discriminant ($D$) of this equation should be less than $0$
$$D=4a^2-4a+1\leq 0\implies(2a-1)^2\leq0$$which is only true when $a=1/2$, but this is not the correct answer!  The least value of $a$ required is $0$ and $f(x)$ is increasing for $a\in[0,\infty[$ but my equation says it increases only if $a=1/2$
So where am i going wrong?  Thanks in advance! 

A:

So where am i going wrong? 

In the following part : 

$$\implies2y^2+(2a+1)y+a\ge0$$ So discriminant ($D$) of this equation should be less than $0$

Note that $y\ (=e^x)$ is positive.  So, what we want is the condition for $a$ that $2y^2+(2a+1)y+a\ge 0$ holds for every positive $y$. (That you had $D\le 0$ means that you found the condition for $a$ that $2y^2+(2a+1)y+a\ge 0$ holds for every $y$, including non-positive $y$).  
We can write
$$F(y):=2y^2+(2a+1)y+a=2\left(y+\frac{2a+1}{4}\right)^2-\frac{(2a-1)^2}{8}$$
The vertex of the parabola $Y=F(y)$ is $(-(2a+1)/4,-(2a-1)^2/8)$ where $-(2a-1)^2/8\le 0$.
We have to have $$-\frac{2a+1}{4}\lt 0\quad \text{and}\quad F(0)\ge 0\iff a\ge 0$$
and this is sufficient. 
$\qquad\qquad\qquad$

