Answer
$c-\frac{b^2}{4a}$
Work Step by Step
For $a\gt0$, the least value of $f(x)=ax^2+bx+c$ is the y-coordinate of the vertex.
We have $x=-\frac{b}{2a}$ and $f(-\frac{b}{2a})=a(-\frac{b}{2a})^2+b(-\frac{b}{2a})+c=c-\frac{b^2}{4a}$
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