Question

The graph of a quadratic function is represented by the table.

Х
f(x)
6
-2
7
4
8
6
9
4
10
-2
What is the equation of the function in vertex form?
Substitute numerical values for a, h, and k.

Answer 1

f(x) = -2(x - 8)² + 6

Explanation:

I did the test.

Answer 2

The equation of the function in vertex form is f(x) = -2·(x - 8)² + 6

Explanation:

The given values are

x, f(x)

6, -2

7, 4

8, 6

9, 4

10, -2

The equation of the function in vertex form is given as follows;

f(x) = a × (x - h)² + k

To find the values of a, h, and k, we proceed as follows;

When x = 6, f(x) = -2

We have;

-2 = a × (6 - h)² + k = (h²-12·h+36)·a + k.............(1)

When x = 7, f(x) = 4

We have;

4 = a × ( 7- h)² + k = (h²-14·h+49)·a + k...........(2)

When x = 8, f(x) = 6...........(3)

We have;

6 = a × ( 8- h)² + k

When x = 9, f(x) = 4.

We have;

4 = a × ( 9- h)² + k ..........(4)

When x = 10, f(x) = -2...........(5)

We have;

-2 = a × ( 10- h)² + k

Subtract equation (1) from (2)

4-2 = a × ( 7- h)² + k - (a × (6 - h)² + k ) = 13·a - 2·a·h........(6)

Subtract equation (4) from (2)

a × ( 9- h)² + k - a × ( 7- h)² + k

32a -4ah = 0

4h = 32

h = 32/4 = 8

From equation (6) we have;

13·a - 2·a·8 = 6

-3a = 6

a = -2

From equation (1), we have;

-2 = -2 × ( 10- 8)² + k

-2 = -8 + k

k = 6

The equation of the function in vertex form is f(x) = -2·(x - 8)² + 6