2 questions regarding parabolas, formulas, etc.

[georgebaseball]

1) How do I know when should I use the formula "y = (4ac - b^2)/(4a)" and when I should use the formula "x = -b/(2a)"?

I'm very confused about that

2) My professor ask us to express in standard form the following quadratic function by completing the square:

f(x) = a(x - h)^2 + k

Then he asked us if the if the parabola opens upward or downward. I suppose it opens upward...?

Thank you for your help!

 

[stapel]

1) You should probably use the "x=" formula when you're trying to find the x-value, and use the "y=" formula when you're trying to find the y-value.

2) But f(x) = a(x - h)² + k already is in completed-square form. What manipulation was he wanting you to do with this...? Also, without a specific value for "a", or at least knowing its sign, there is no way to know which way the parabola is going to open. Sorry.

Eliz.

 

[skeeter]

1) How do I know when should I use the formula "y = (4ac - b^2)/(4a)" and when I should use the formula "x = -b/(2a)"?

x = -b/(2a) is used to find the x-value of the parabola's vertex.
y = (4ac - b^2)/(4a) is the y-value of the parabola's vertex.


I'm very confused about that

2) My professor ask us to express in standard form the following quadratic function by completing the square:

f(x) = a(x - h)^2 + k

Then he asked us if the if the parabola opens upward or downward. I suppose it opens upward...?

if a > 0, the parabola opens upward ... if a < 0, the parabola opens downward.

Thank you for your help!