Question

The line of symmetry for the quadratic equation y = ax^2 + 8x - 3 is x = 4. What is the value of "a"?

-2?

Answer 1

a determines the "shape", the steepness of the curve. OK?

Answer 2

yep

Answer 3

The line of symmetry is (I believe) the line that splits the parabola in two equal parts, here the vertical line that goes through the vertex. OK?

Answer 4

So the question is, do you think that knowing the steepness of the curve can tell you anything about its vertex, or alternatively, knowing the vertex can tell you anything about the steepness?

Answer 5

OH.. screwed up, of course

Answer 6

vertex is at x = -b/2a = 4
b = 8

Answer 7

?? i dun get it can you explain it more easier plz

Answer 8

There is an equation you need to memorize.
At the vertex (and along the line of symmetry), x = -b/2a.

Answer 9

ok

Answer 10

We're given x and know b from the equation, so we can compute a.

Answer 11

ok

Answer 12

We have
x = 4 = -b/2a = -8/(2a) = -4/a

Answer 13

y=ax^2+8x-3
y=a4^2+32-3
y=a4^2+29
y=(1)4^2+29

Answer 14

Huh?
a = -1
Check by
y = -x^2 + 8x -3
vertex is at -b/2a = -8/2(-1) = 4
check

Answer 15

oh y=(-1)4^2+29

Answer 16

y=13

Answer 17

Good. But the question only asks for a.