Question

Find the minimum value of the function f(x) = x² +10.5x + 20 to the nearest
hundredth.

Answer 1

well, the quadratic function is simply a parabola that looks like a bowl with a "minimum" at its vertex, so

`\textit{vertex of a vertical parabola, using coefficients} \\\\ f(x)=\stackrel{\stackrel{a}{\downarrow }}{1}x^2\stackrel{\stackrel{b}{\downarrow }}{+10.5}x\stackrel{\stackrel{c}{\downarrow }}{+20} \qquad \qquad \left(-\cfrac{ b}{2 a}~~~~ ,~~~~ c-\cfrac{ b^2}{4 a}\right) \\\\\\ \left( -\cfrac{10.5}{2(1)}~~,~~20-\cfrac{10.5^2}{4(1)} \right)\implies \left( -\cfrac{10.5}{2}~~,~~20-\cfrac{110.25}{4} \right) \\\\\\ \left( -5.25~~,~~20-27.5625 \right) ~~ \approx ~~ \boxed{(-5.25~~,~-7.56)}`