Question

The line of symmetry of the parabola whose equation is y = ax^2 - 4x + 3 is x = -2. What is the value of "a"?

a) -2
b) -1
c) -1/2

Answer 1

The line of symmetry occurs at that line of x where the parabola is a minimum (or maximum). The x-value at which a parabola is minimal (or maximal) is \[x=\frac{-b}{2a}\]for a parabola with equation, \[y=ax^2+bx+c\]
You have the following,\[x=-2, b=-4\]Solving for a gives you\[a=\frac{-b}{2x}=\frac{-(-4)}{2(-2)}=-1\]which is answer (b).